For a few years, I was suffering slightly from Carpal Tunnel Syndrome. This is self-diagnosed, of course, I never tried to get treatment for it. Can't go to the doctor for every little thing...
Instead, I tried switching mouse-hand, but that only moved the problem between my hands. I used various support hand braces, and while that never cured the problem, it would take away the symptoms momentarily. For a while I even tried monkey-footing a track ball - with some success, might I add.
Last autumn, somebody on an internal mailing list suggested that I try the Humanscale Whale Mouse. It's a much larger mouse than what you're used to. The manufacturer says that it's:
Ergonomically designed to fit your hand and take the stress out of mousing. Traditional mouse designs force you to grip the mouse between your thumb and little finger. This strains the more delicate muscles in your hand and wrist. The Whale is designed to relax your hand. This allows the larger, stronger muscles in your arm to handle the stress of repetitive movement.
I ordered one of these even though it seemed pricey and even though I wasn't impressed by the specs of it - no scroll wheel (maybe intentional?), and it's opto-mechanical so the mouse ball picks up a lot of desktop gunk. Still, at least it's USB.
I've now used it for about six months. I've only used it at work, and only on one of the machines (my main machines). On all my test machines and at my computer at home, I've been using the normal Microsoft optical mice. By now, I haven't noticed any wrist or hand problems for at least a couple of months. In fact, I had forgotten all about it until just now.
In my case, it sure performed as advertised, and I think that it was well worth the money. I do however hope that there will be an updated version of it - all optical and with a scroll wheel. In the mean time, I'll happily clean out the mouse ball every couple of weeks.
In case you're interested, more information about the mouse is available at http://www.humanscale.com/products/whale_mouse.cfm.