If you have ever had pain in your mousing wrist/arm, I suggest you run (do not walk) and order yourself an Evoluent Vertical Mouse. Three buttons + scrollwheel/button + thumb button, nice and light, fast fast tracking, and fully programmable in OS X using USBOverdrive. It is so incredibly comfortable. Sadly the Canadian distributor I ordered through (ErgoCanada) erroneously sent me the black/black version rather than the super-sexy purple/black version, but I love it way too much already to bother sending it back.
I call mine FATMOUSE.