I've been on trackballs since the Atari ST days! Give me a static hand position, and a ball the size of a cue-ball (so you don't have to have a really precise thumb motion, you use your index and middle fingers on the ball, and thumb and fourth finger on the buttons) and I can work it all day long with zero fatigue.

I highly recommend the Kensington Turbo Mouse Pro... Wireless, and a plethora of buttons you can configure any way you want. Awesome ergonomics, once you make the move...