Apple mice really do not form to the human hand at all. But it hurts my hand less to use it than others...
Last year I was using a nice Logitech mouse, I loved it, I had a button for expose, dashboard, safari functions, and I was able to use functions of OSX much more efficiently than before. It also fit my hand very well.
Soon I developed pain in my right hand/wrist/forearm. All that functionality led me to overuse my right hand, and I looked at using trackballs, couldnt get used to it and didn't help stop the pain. (Flicking of the wrists to move the ball didn't help). I finally ended up getting a cheap wacom tablet and using the pen until the pain went away. Now I alternate between that and my apple pro mouse (non mighty).
I find that, although it is unergonomic and only has one button, it hurts my hand less. That is because it forces me to use more keyboard shortcuts, quicksilver actions, and sometimes even voice commands, and my actions are evenly distributed through different methods of input rather than clicking away.
Just some thoughts...
Oh and if you do really want an ergonomic solution, a wacom tablet works wonders (after the learning curve). You can also get this ergonomic mouse (the only truly ergonomic one, since the hand is in the natural sideways position like holding a joystick) by 3M.
http://www.amazon.com/3M-Ergonomic-...1/ref=pd_bbs_sr_1/104-2195202-4417537?ie=UTF8
I didn't get the 3M because it wouldn't work with my desk placement (doesn't fit in the keyboard/mouse tray.