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...

It's certainly less stressful on my wrists for me. I can now move the pointer all over the screen but just wriggling two fingers over a range of about an inch. My wrist and arm are always in a resting position when trackballing, and left clicking with my thumb feels easier than with my forefinger.

I tried those for a while when I was on my last ergo kick. The ball was just too hard to move for me. I almost had to pinch the thing to make it slide.

Instead, I've found this more comfortable: http://www.evoluent.com/vmouse2.html

Works great with a gel wrist wrest on the mousepad.

i like neither, i prefer the 'nipple' (ibm and fujitsu laptops). touchpads are nice too.

i got physical pains from reaching the mouse