I use one
I use one with my C2D unibody when booted into Windows to play games.
Most recent games tax that machine so much, it'll shut down after about an hour of game play. The Targus chill mat I use will keep my machine up for as long as I care to play.
Don't use it when booted into OS X, but I don't think I stress the machine while booted in OS X like I do when gaming in Windows.