Archons can attack both ground and air units and its attack causes minor splash damage (50% within 15 pixels, 25% within 16-30 pixels). Attacking groups of enemy air units greatly increases the efficiency of archons.
An archon has most of its "health" invested in its shields. This means that they take full damage from all attack types and are vulnerable to terrans who have the science vessel's EMP Shockwave. With such a reliance on shields, shield upgrades at the Forge as well as shield batteries are a great boon for heavy archon use.
Unsurprisingly archons are expensive, especially in gas. Each effectively costs 100 minerals, 300 gas, and 4 psi, the combined price of the two sacrificed high templar. The cost necessarily limits the number of archons that can be used, ensuring that lesser units like zealots remain a vital part of protoss strategy.