Zerglings have been reduced in size and have much better path-finding than in StarCraft. Perhaps to balance this, the zergling's attack possesses a much higher cooldown in relation to the marine and zealot than it had in the original StarCraft. An unupgraded zergling was able to attack almost twice for every time a marine fired, and had nearly a third of the zealot's cooldown. In StarCraft II an unmodified zergling cannot match this attack rate, and even with the Adrenal Glands upgrade the zergling can only strike twice for every zealot attack.
Zerglings are very useful for an early rush. If the opponent masses units or builds defensive buildings you can morph some of them into banelings, a superior choice against some unit types. Due to their cheap cost and high speed, zerglings are ideal scouts.
Zerglings are useful even into the late game due to their ability to surround opponents quickly.
As of May 2011, the most common early-game zergling tactic is to set them up at xel'naga towers.
In Heart of the Swarm, zerglings are available starting mission Lab Rat, and later can be upgraded into either raptors or swarmlings. At Level 10. Kerrigan can be given the Zergling Reconstitution ability, which causes killed zerglings to respawn immediately at the player's primary hatchery, up to 10 zerglings every 30 seconds. Zerglings are available on the first mission of Heart of the Swarm campaign.
Upgrades and Abilities
Increases zergling HP by 10.
Tissue testing has revealed beneficial mutation of zergling plating. Carapace can be hardened in key locations throughout thorax. Will provide significant increase to life expectancy, hamper access to vital organs.
Adrenal functions of zerglings simple to mutate. Often modified on battlefield through spawning pool. Possible to improve adrenal system at base level, increase glandular output. Will overload adrenaline production, greatly improve zergling attack speed.
Bolstering metabolic system of zergling, common battlefield strategy. Possible to augment enzyme production, improve energy production rate. Will provide significant increase to movement speed, allow zerglings to close distance on foes outrun pursuers.
Zerglings are available to Zagara and Kerrigan. Their zergling strains automatically evolve when they reach Level 12: Zagara gains swarmlings, and Kerrigan gains raptors. The two unlock zergling upgrades at certain commander levels: Kerrigan unlocks the following upgrades at Level 4, and Zagara acquires them at Level 6. 
In multiplayer, a mutant skin is available for zerglings that is unlocked upon reaching level 30 with zerg. It uses a similar model for swarmlings for base zerglings and raptors for zerglings with the metabolic boost upgrade.
↑Zerglings are always useful in late game due to their speed and ability to surround enemies. Banelings too are especially useful against both Zealots and Marines, even with their upgrades, and thus will always be useful to counter them. Additionally, Banelings are always useful in raiding, which should be done throughout the entirety of a StarCraft II game. Karune. 2009-03-16. Teir 1 Unit in Late Game. Battle.net StarCraft II General Discussion Forum. Accessed 2009-03-16.