After prestiging all 3 times with Raynor and getting very comfortable with marines and BCs, switching to Kerrigan was jarring to say the least. It almost felt like Raynor was SC1 while Kerrigan is more like WC3, with a hero unit that is the main focus and friendly units for support. And no, I did not play HOTS which is why this probably sounds ridiculous.
Kerrigan functions as an early game tank with relatively poor damage. Her assimilated resources and mobility through worms makes her an extremely useful supportive player.
Kerrigan can function as a solo with randoms if need be, but functions much better with a cooperative ally that listens to her when she gives callouts for immobilization/assimilation. As a solo, she needs to be aware of the fragility of her units, fighting smaller obstacles by herself to minimize needless hydralisk/mutalisk deaths.
Benefits to other commanders
Kerrigan's biggest boons to her allies are her creep, which increases attack rate, movement rate and regeneration (making her an automatic ally for practically any zerg), and her assimilation aura. Her assimilation aura is far better on harder difficulties, since on normal you might find groups of five units while on brutal difficulty that number is more like 30. An experienced Kerrigan can tank for an allied early game army and really kick both players' early economies into high gear with this alone. Her assimilation is particularly amazing with her third prestige. Overall depending on the map, as well as how cooperative your partner is, you can expect about 1-5k resources assimilated for each of you. This is NECESSARY for Kerrigan due to the very high unit turnover and multiple upgrades she has, but for other commanders that are otherwise used to not working with extra resources, they can quickly convert the money into maxing out earlier.
Kerrigan's creep bonus is particularly potent with Abathur and especially Stukov, who spread a bunch of creep around themselves. Those familiar with multiplayer might instinctively go for queens and their tumors, which can be valid on defense maps. But omega worms spread creep much more easily, as they can be called upon to make creep instantaneously anywhere on the battlefield. Ditch the queens unless you are specifically playing with P2.
The Nydus Network
I'm writing this down because it might be confusing to wrap one's head around. The Nydus Network is a building that you can morph after getting a lair. Once it is built, it can spawn a head anywhere on the map that you have vision, allowing units to travel freely between the two. Once it spawns a head, it can spawn another after a cooldown. Nydus networks can spawn a theoretically infinite number of heads. Units can travel between any head (or butt). Building multiple canals means that you can spawn heads much more often. Nydus worms upgrade to Omega worms at Kerrigan level 7ish. Omega worms summon additional heads for free, unload units much faster, act as detectors, are harder to kill, and instantaneously burst from the ground rather than the Nydus worms' long "build" time.
In addition to providing incredible mobility for anywhere on the map, Omega Worms also spread creep and act as detectors. They also have 1,000 HP, making them great meat shields. The instant creep spread is highly beneficial when pushing into locations, ensuring that at least some of your units benefit from the additional attack speed and movement rate during the battle.
Once you get a lair, build a Nydus canal in each of your bases. Have all hatcheries/lairs in each base set a rally point to each of their networks. Now, whenever you train units, they automatically head into the network, allowing you to pop a worm anywhere for them to bring them immediately into the fight.
Your allies can use the nydus network, though some commanders may find this not too useful (like Tychus).
Prestiges and Specializations
Standard Kerrigan will want to go with hydralisks, with liberal use of Omega worms. Kerrigan clears the early waves while hydras provide late game firepower. Throw in zerglings as a mineral dump.
Exceptions to the rule exist. Zerglings perform incredibly well for their cost with an Artanis thanks to guardian shell. Ultraling can also be viable, though it suffers in choke points. I've seen mutas work well if the player is attentive enough to avoid AoE.
Creep bonus are increased by 100%, which is insanely good. You lose the ability to build worms, which is insanely bad. To make up for the loss of mobility your queens are better and spread creep wherever you want.
This is naturally amazing with commanders that either spread their own creep, like Abathur or Stukov, and is ok when paired with those that are highly mobile, like Alarak or P2 Artanis. It's also good for defense maps, though even in those cases omega worms are still much faster.
One significant problem is that aside from when paired with Stukov, the creep bonus is really only usable for defensive situations. You can't preemptively push creep past an enemy position you're about to attack, no matter how gosu your tumor spreading is. Omega worms CAN spawn creep out of nowhere, and regular creep bonus is a lot better than having none at all. Conceivably, you can try to make use of the other main benefit (queens are now much more mobile) so that they can continually transfuse ultras and kerrigan...but you can do the same by loading queens into worms by default. Even with the mobility advantage of double-effective creep, your army will still be far less mobile than with worms.
For laughs, try pairing the creep movement bonus with Stettman's movement bonus or Raynor's drop speed+stim on his marines.
Minor side note: This prestige is undoubtedly best for a tiny window, the level between getting malignant creep and getting omega worms.
Folly of Man
Whenever Kerrigan uses her Psionic Shift or Leaping Strike, she uses up all her fury stacks (the same fury she gets at level 9), stunning and damaging enemies in a large area. At max fury, she can hit 24 enemy units at once. The damage is sufficient to instantly kill smaller units, and severely damage most but the tankiest units. Kerrigan becomes nominally frailer, generating carapace (her extra temporary health) 50% slower. She deals 50% less damage with her abilities.
This prestige is arguably a straight upgrade from the default. The reduced carapace generation nominally makes Kerrigan a worse tank, but this is more than made up for by the fact that every few seconds you can stun practically the entire enemy army, giving her time to heal up and generate more carapace.
Doubles resources from assimilation aura (incredible). She loses psionic shift and her leap, instead gaining Crushing Grip and Kinetic Blast. These greatly change Kerrigan as a unit. She loses the mobility of her leap, but instead gains an ability that deals roughly the same damage but at a safe range, allowing her to pick apart enemy armies without even aggroing them that much. Her crushing grip hits theoretically fewer targets than psionic shift, and cannot be used as a mobility tool, but both her abilities can now target air units, removing one of her biggest weaknesses.
Overall, this version of Kerrigan handles ground waves somewhat more poorly, particularly large swarms of small units, but becomes much better at kiting and dealing with air units.
- Kerrigan Energy Generation or Kerrigan Attack Damage: the former has better damage output but the latter is better if you don't have time to micro her. SC2 coop recommends a 22/8 spread so she can 3 shot banshees; I don't think it's a huge deal.
- Combat Unit Vespene reduction or Augmented Immobilization wave: The former save you a huge amount of vespene over time, allowing you to dump it on more units and upgrades. The latter increases the damage, but not duration, cooldown, or area of effect, of the immobilization wave. One can argue that this is BAD because the wave's radius is far larger than assimilation aura, meaning that you waste resources if you wipe out a bunch of dudes outside of the assimilation bubble. I personally put all points in combat unit vespene, allowing me to get my death ball up much faster and dealing much more consistent damage than the immobilization wave.
- Expeditious Evolutions Primary Ability Damage and Attack Speed: The former increases the speed of research and decreases their cost, while the latter increases Kerrigan's damage output. Now this one is kind of interesting. I'm about level 70 right now, and dumping all points into unit vespene reduction AND evolutions has left me with more gas than I can use before I max out on most maps. A 20/10 spread might be enough to keep my gas use efficient while giving Kerrigan a buff.
- Kerrigan: you should be using all of your early larvae on drones to get a good economy ASAP, which effectively means that Kerrigan is your only early game army. But what an army! She can wipe early waves solo with ease while giving you and your ally respectable doses of resources. Spam her moves as often as possible, and try to get enemy armies to bunch for maximum effect.
Kerrigan pales a bit in the late game, particularly against air units. Toward the mid game though, IF you can solo something with her, do so. Only use the rest of your army when things start getting hairy, and you will save yourself a lot of needless losses in the long run.
- Zergling: A mineral dump. Very high DPS is tempered by the fact that they die practically instantaneously. They are also good at clearing out unprotected buildings, though all units are good at that anyway. A couple of zerglings in the early game to clear rocks is not a bad choice. Works great in conjunction with immobilization wave and omega worm.
- Hydralisk: Your bread and butter. Great DPS but fragile. Keep them behind Kerrigan and your allies, and rush them to Nydus worms when a bunch of them take a lot of damage.
- Lurker: Great siege defense unit. Don't bother on offense, too slow and micro intensive. Good on the evacuation mission and the building clearing one at night.
- Ultralisk: Get a couple--more than 5 and they will cause a lot of congestion and possibly more harm than good--and you will have a very capable meat shield for your hydras. Notably, ultralisks are the *only* unit type aside from Kerrigan herself that can take a hit, making them useful for mutations that do a lot of damage.
- Queen: Build just two (one for each base) to inject larvae occasionally. They need too much APM to use when your attention might be taken up by more important things. Creep spread is redundant because of worms.
- Mutalisk: Great DPS, but fragile and incredibly vulnerable to splash damage. Great against certain AI compositions, but beware psi storm, parasitic bomb, fungals, BCs, etc. Incredibly gas expensive too; you will end up with a ton of minerals, so build more hatcheries and spam zerglings. Mutalisks actually have better DPS than hydralisks since they don't need to worry about clumping at chokepoints or shuffling past each other, making them the clear choice for maps that lack significant anti-air.
- Brood Lord: Slower DPS, but a good deal safer to use. If you have an ally with an incredible ground army, this can safely provide support without blocking them. Even more wildly gas expensive than mutalisks. Really only viable for massing with a Swann teammate and/or maps where P3 Kerrigan can assimilate tons of resources/take expansions. It's harder to get Brood Lords killed than any other unit in Kerrigan's army, but losing them is catastrophic, so be careful for them.
Kerrigan benefits from communication more than most commanders. Her assimilation aura and her immobilization wave both work best with the ally right next to her. Allies that are intent on ignoring her and doing their own thing not only hurt her, but to a lesser extent themselves. A well-played Kerrigan can provide very high DPS, mobility, and free resources for her team, but one that is unable to coordinate with her ally will find her frail army quickly evaporating time and time again. Allies can help with their own damage, or even better, absorbing damage and using abilities to augment her army.
The most viable builds for Kerrigan are:
1. Mass hydralisk: rely on the mobility and healing of the network to make up for hydra shortcomings, and use Kerrigan to tank, popping them out of networks for DPS. This is one of the best all-round builds and is viable against any enemy composition except heavily anti-ground ones, and even against those, aggressive nydus worm placement can help them overwhelm strong enemy positions.
2. Ultraling: very powerful and cheap--but only viable with Artanis, otherwise the lings die before they can get any damage in. Build extra hatcheries because even with Artanis' guardian shield and shield overcharge, you'll be losing zerglings like crazy and find yourself constantly out of larvae otherwise. Extra nydus canals are also useful due to the sheer numbers in your army causing congestion. This build is ideal against zerg for a couple of reasons: first, the creep makes your melee units close the gap and deal damage faster, and secondly also because this is an extremely mineral heavy composition and your assimilation can quickly replenish minerals thanks to farming lings.
3. Mutaling: dump all gas into mutalisks and their upgrades, using zerglings as a mineral dump so that Kerrigan doesn't get overwhelmed on the ground. This is Kerrigan's highest DPS build, allowing her to wipe out entire bases within seconds. However, it is extremely fragile to enemy AoE: storms, irradiate, thors, liberators, and parasitic bombs can wipe out half of your mutalisks within seconds, so good micro and awareness are needed against certain unit compositions. In practice, this is by far the best build against any protoss composition: you can flit out of storms quickly enough, and any damage is quickly healed. It is also probably the strongest composition against nearly any zerg composition, with the exception of ones that have vipers. Using this against terran is risky: if they have thors and science vessels, use hydralisks instead, and even if they don't explicitly say they have liberators, thors, science vessels or ravens, oftentimes they'll throw them in anyway.
Counterintuitively, Mutaling is less mobile than mass hydra due to the fact that they can't use worms. Some maps demand mobility more than others; hydras are stronger for them.
Don't bother with brood lords, which are far more expensive than mutalisks yet have poorer DPS, or queens, whose functions are entirely redundant thanks to omega worms and malignant creep.
When doing a non-muta build, go for 3 hatcheries->two gas->fourth hatch at expo and take gas immediately->spawning pool and 2 evolution chambers, building drones whenever possible and overlords whenever close to supply capped. Upgrade Kerrigan as soon as possible, using her to carry the first waves until hydra/ultraling production starts up.
When doing a muta build, gas is more at a premium, so get two extractors immediately after getting the first hatchery, then proceed as normal (just one evolution chamber is necessary). Try to snag extra extractors (Oblivion Express, Void Launch, a teammate who is not using their natural's).
Certain enemy builds practically completely shut down any Kerrigan unit composition, as far as I can tell. Zerg comps that are heavy on vipers near-instantly kill any mutalisk clump thanks to parasitic bomb, and simultaneously makes hydralisks useless due to their cloud ability...this leaves Kerrigan with zerglings, who die instantly against anything unless she has an Artanis, and ultras, who clump and deal bad damage. On the terran side, the siege tank/bc/science vessel/thor combo is a bit too thick for hydralisks to chew through, and irradiate/thor kills mutalisks very, very quickly. Nominally, the best way to deal with this is to avoid waves until immobilize is ready, or have Kerrigan pick off individual deadly units before sending in the army, but this can be tricky.