if your protoss and your fighting zerg, usually the opponet keeps their overlords packed together in a particular area...if you make corsairs quick by making a gateway, cybotonic core then a stargate and a fleet. you'll be able to get it and plus corsairs are produced pretty quick so while your defending your base you move your corsairs in and take out there overlords and then you move in for the kill while he's still making overlords.

CreepColony's Opinion: Corsairs are awesome for taking out mass overlords. Build like 12 and attack, and if they try to stop you with hydras, just disruption web them.