I use the older, bulkier version of the Intellimouse Explorer for gaming and everything else I do. I know the newer one has a higher frequency (i think that's what you call it). But I like the older one because of the bigger buttons on the side.
It works great for everything I do, including CS, UT and countless other games. At first I had trouble getting used to it, because the way it tracks is a little different from what I was used to with a ball, but now its great.