Hi Akoko, I'm using Logitech Trackballs for about 8 years now. Playing WOW with it since 2 years. I started dual hardware boxing about a year ago and used two trackballs and two keyboards, which worked quite fine, as the trackballs did not move position and I could alternate between them without looking. But nowadays I'm running all my toons on a single machine.

About half a year ago i searched for alternatives to my logitech ones, but did not find any. There are two points, where current trackballs can't compete with mice: Resolution and number of buttons. I can live with the first, as I use the trackball mainly for movement. I can even do turns in the air while jumping with my mage and hunter. If you want it to use for targeting, you might miss the precision though. The low number of buttons is the more annoying thing to me, as when using the thumb, you have four fingers, which could potentially trigger events. If you neglect the middle button, which is hidden beneath the scroll wheel, i can currently only use two of them.

Concerning RSI I read, trackballs might not really help per se. They can be used alternating with mice to avoid the same hand position. If they're used as the only device, you probably won't have any benefits.