I didn't mention that the mouse I'm using now is THE best mouse I've ever used and I've not had the slightest twinge of RSI in over 18 months since I've had it.
It's a R.A.T. 7 and really a gaming mouse with a couple of buttons I never use (not sure where to use the sniper button in excel).
Two reasons I like this mouse. One, it's adjustable, it comes with extra parts which you use to fit the mouse to your hand. I have shovel size hands and I've adjusted this mouse to fit nicely in my hand. And two, sensitivity, this mouse has a crazy DPI of 6400 which means I hardly have to move the mouse to cover the 2 30" monitors I have on my desk. It does take some getting used to, but I've found I never have to move my hand more than about 2" to cover the whole desktop area. Also, the DPI is adjustable on the mouse, so if you are focused on one piece of the screen you can drop the DPI and slow the mouse down.
The there's build quality, which is top notch, all buttons are light and large, good scroll wheel (though not free spinning which would be nice) and you can even adjust the weight as it has removal set of sliding weights.