I have a MS Wireless Intellimouse Explorer, and frankly, it sucks. Sure, it fits my hand alright, but:
1) New batteries cause it to emit a squealing sound for 1-2 weeks (until the voltage drops some from use).
2) Moving the mouse any faster than a normal motion (say, to spin around to shoot someone behind you) causes the cursor to violently shake from left to right (so that you can't turn all the way around, instead you just look left and right repeatedly).
3) The range on the mouse is greater than 15ft., so anyone near you with anything on the same frequency will cause your cursor to go nuts until you change the channel.
Needless to say, I'm gonna get something else soon. Oh, I used to use a MS Wheelmouse optical (the basic one with two buttons, wheel, and optical), and that worked great for FPS games. No complaints about that one except that it was kinda small for my hand.