I have the Mighty Mouse, and it is an ergonomic nightmare. It turns my hand into The Claw after about an hour or use. I've gotten used to the right-clicking thing (not that you really need to right-click all that often in OS X), but it's still far less efficient than a standard mouse.

I own the Mighty Mouse and I can't stand it. It wouldn't be so bad if it would consistently register a right-click. Unfortunately, half the time when I hit right-click, I get a left-click instead. I love the multi-direction scroll, but independent physical buttons are far more precise.