I actually have an Evoluent VerticalMouse3. I got it because I suffer from tendonitis.
I switch back and forth between the VM3 and a Contour Perfit (optical) mouse
because I find that both tend to relieve tension in one area (e.g., the radial nerve) but cause tension in another (e.g., the palm, or the back of the hand).
For the Contour Perfit, there are different sizes and different models for right vs. left-handed use (I use a large right-handed one). They're designed so that your whole hand rests on the mouse, in a "neutral" position (halfway between full flexion and full extension).
I think I like the Contour mouse a little better. I find with the VM3, despite the (very slight) lip at the bottom, my hand still tends to drag on the mousepad.
The scroll wheel, on pretty much any mouse I've tried, is a major culprit for repetitive stress injury. But the function is just too useful to give up!
Probably a bigger problem is the one discovered by rehtonAesoohC (parent poster): fine mouse movements are very difficult to achieve when using your whole arm rather than your fingers. Those larger muscles just lack the precision of our fingers, which is why I often find myself using the Contour mouse inappropriately, with my fingertips touching the mouse and rest of the hand arched up above it rather than laying flat. I've developed a few little tricks to get back some precision, like incorporating a kind of wrist wiggle to move the mouse left and right in a fan-like motion and get the mouse pointer to home in on a specific little point. But that can lead to wrist pain if overdone, and I think especially gives me pain along the radial nerve when using VM3
At any rate, I'm glad to see other recommendations from Slashdotters, and may try a few more mice. Don't overlook the possibility of using several mice and switching between them at times.