i use 2 since its far easier to have multiple things open then with one.
think about it, how often do you switch back and forth between a couple different windows or open programs? if you have 2 screens, then you can better spread the things around to reduce the time spent finding one if the windows.
and i have to agree with what codeman said. once you experience more then 1 monitor, you wont want to go back. getting used to 2 monitors then going back to one feels like getting used to high speed internet then going back to 56k, it feels so ungodly slow, whereas before you even got the high speed internet, the 56k was tolerable.
i'm actually considering getting a 3rd screen. then i can have the primary in the middle and 1 on each side.