Using Counter Strike as the flag ship FPS for Valve there currently aren't any professional players using a trackball: https://docs.google.com/spreadsheet/pub?key=0AkghPvIh7ZVwdGZ...
the thumbs trackball is more precise. But still not good. A good and cheap microsoft mouse is always with me when i'm playing FPSs.
Also, even if the control wasn't an issue, no trackball has more than 300DPI. EVER. The minimum for mouses is 400. I don't know why that is. But even the overly expensive Kensington ones are crappy even to click on a 10x10 button on regular usage, let alone to headshot in a FPS.
My 1600DPI L-Trac disagrees.
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.
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.