stack is great for dealing with space isues caused by supply depots/pylons. it also works great for hatcheries cause u get tons of larva in the same area.

as sov said there is a counter but ive never heard of stack hack affecting the outcome of the game. now if u could say stack units and move them at the same time that would kill totally (40 tanks on top of eachother seiging?)

one of these days im gona get up and scream "stop being stupid!" then everything would be alirght. For seriously.