I don't know. I haven't tried the Magic Mouse. I disliked the Mighty Mouse's scroll ball implementation, but loved the concept. The small ball gets dirty to easily.
I find I miss the gestures I can do on the multi-touch pad on the MacBook Pro when I'm on another notebook... but I haven't yet tried the new Multi-Touch track pad. I think, given my appreciation and missing it when I don't have it, I will like it.
I use a Logitech TrackMan Marble Wheel for my desktop... and just bought a new one because my ten year old one is getting shabby looking.
My wife uses the magic mouse as a track pad...she and the kids keeps switching the settings. I haven’t used it much, so I don’t know exactly what they are doing...they both love the mouse, but use it very differently.
The scrolling on the Magic Mouse is far better than any mouse ball or wheel I’ve ever tried, plus I get limited gestures. My dislikes are that I lost the middle button, and the edges of the top should be a little less sharp, more rounded.