I wholeheartedly agree that trackballs suck for FPS games, but most pro FPS players advocate keeping the mouse at a fairly low DPI, so that deficiency isn't really the problem.
I have trouble believing that any device that relies on thumb-only input is going to be anywhere near as good as a mouse, but I hope I'm proven wrong.
Trackballs really aren't _that_ terrible for precision in FPS. Not pro level, but heckuva lot better than the joystick control consoles currently use. I found I had a harder time in StarCraft than I did in UT2K4, but both were quite playable.
I used a Logitech Trackman for seven years, and the biggest issue was that the force required to move the ball around wasn't constant; it would vary depending on how gunked up the little rollers inside had gotten. I had to clean mine out every day to keep it smooth.
If a person prefers using a trackball, that's great, but when the same tools used by the pros costs about $50, there isn't really any other reason to use anything else.
I have trouble believing that any device that relies on thumb-only input is going to be anywhere near as good as a mouse, but I hope I'm proven wrong.