I like the trackball for three reasons:

- I don't have to periodically pick it up and reposition it because of running out of mousepad room. (My wrist doesn't like doing that, and my sister -- who has had surgery for carpal tunnel syndrome in both of her wrists -- says it's bad too, but I couldn't find an online citation.)

- I can press the buttons without accidentally moving the pointer at the same time. (This is mainly a problem with ball mice, but I've gotten unintentional pointer movement with optical mice too.)

- I find it easier to do things that require pixel-level accuracy with a trackball.

I prefer the Logitech TrackMan Wheel, whose trackball is thumb-operated:

http://www.logitech.com/index.cfm/mice_pointers/trackballs/d...