I also have used Trackballs for several years and use them for gaming too. I love it! People think I'm crazy when they hear I'm playing FPS games with a trackball, but I think it's probably better than a normal mouse, because you don't have to constantly move your arm, pick the mouse up and move it back, and all that.
Also, when you get really good with it, you can kinda just give it a quick spin and position your gun where you want it more quickly than with a normal mouse.
I think you'll enjoy it! With trackballs, you either love em right off the bat, or hate em.
I use a wireless Kensington Turbo Mouse Pro, which has a ton of buttons I can program to do different things in the game, like selecting certain weapons/grenades/etc.