There are two main ways to hold and use a mouse:
1. Rest your whole hand and palm on the mouse and move the mouse in sync with your hand.
2. Hold the mouse by its sides, leaving a space between your palm and the mouse. That way you can do precise movements by moving the mouse with your fingers, only moving your wrist/arms for wider movements.
Apple mice are made to be used with technique #2. People that try to rest their whole hand on the mouse will find Apple mice to be too small and uncomfortable, but that's not how it's supposed to be held.
People that are used to technique #1 will prefer relatively big mice that "fits" the shape of their hand perfectly. These folks will look at either the Mighty Mouse or the Magic Mouse, without trying it and conclude that it's not "ergonomic" because it doesn't have this particular hand-fitting shape they're looking for.
I'm not an ergonomic expert, but I would venture to say that technique #2 doesn't cause as much RSI as it puts less strain on the wrist, and that the big hand-fitting mice are preventing you from using only your fingers to move the mouse for precise movements since your hand is "stuck" around the mouse, thus forcing you to use your wrists to make any movement.
Personally, I love the shape of the Mighty Mouse, have no problems with right-clicking and I absolutely adore the little trackball. I like the fact that I can click with all my fingers to register a left-click. I tried many other mice and to me the big scroll-wheels don't cut it at all, even the one with ball-bearings.
That being said the big problem with the trackball is the way it would get dirty and stop working. It's extremely annoying to have to clean it every once in a while. Also, I disabled the side buttons since I was always activating them by accident.
The Magic Mouse is the mice that I waited for, I don't care much for the gestures, but I'm pretty sure that the scrolling will be close to what the track ball provided and even better, without the cleaning problem.
BUT... Where is the middle click? I use it all the time in Safari to open links in new tabs. It's really a deal-breaker to me even though I would love to get a Magic Mouse
