The zealot is a powerful melee unit with as good a rush tactic as the zergling. These can esentially counter all of tier one but not that powerful against much else without turning into the temple zealot. Its got a good attack, health, and sheild. However,
fighting against some tier 2 and tier 3 units will cause a large problem, as tier 2 are ranged and on cliffs, can hurt the zealot from afar, while tier 3 cant even be attacked!
In the newer versions of the game, a second type of upgrade zealot exsits, the warp zealot. Unlike the temple zealot, the warp zealot has a blink charge ability. The blink charge enables warp zealots to blink to the front lines, as well as quickly surround and/or chase enemies down.
Zealots cost 200 minerals (in the current beta) and are the second cheapest teir one merc. It is absolutaly essential to get the charge ability if you arfe using the zealot as your primary merc.