I've been thinking about getting a trackball for a while, to see if I found it any easier to use. However finding a cheap one didn't seem possible - everything was at least £30 even before P&P. However last week I found the Logitech Marble Mouse which despite the name is really a trackball. It came to £18.03 inc VAT and P&P which I deemed a reasonable amount to try it out (I wasn't sure a trackball would agree with me).
So far I've been quite pleased. I haven't found myself wishing for the mouse back. It has 4 buttons but only 3 are easily usable (one ends up under the palm of my hand - if I used the other hand then it'd be the button on the other side that got covered). However that's fine. The lack of a scroll wheel I miss a bit - mainly in JOSM for zooming in and out. I think I can do cunning button mapping to allow it by scrolling the ball, but it hasn't been that annoying yet.
I don't (thankfully) have RSI, but I've read various things that suggest a trackball instead of a mouse is a good way to prevent it. Given that I've found it easy enough to use I think I'll be sticking with it for the moment.