After years of using a MacBook Pro, I felt the need to have a multiple monitor setup with everything that it implies - namely an external keyboard and mouse. I tried an old travel Logitech mouse that was laying around, which quickly started causing wrist pain. After returning 4 different mice for the same reasons, I found that Logitech had just released what looked like a better M570 - the legendary trackball I heard so much about in the past. Bought it, tried it, and the pain went away after a couple of days. If you're suffering from RSI, you should definitely give it a try.
