My magic mouse arrived this week. One of the major issues you'll have when seeing the Magic Mouse is that it is incredibly low. The low profile of the mouse at first seems daunting, upon receiving the Magic Mouse I tried to place my hands in the same position that they would be in if it was there.
It's almost impossible to do so, it hurts. However, when having your fingers on the mouse's smooth surface , the pressure seems to be taken off and the position is quite comfortable.
Another issue I've found is that it doesn't support enough gestures. Whether using a trackpad or a (now called) Apple Mouse, I've always had a shortcut to Exposé. With this mouse, that shortcut doesn't exist and it's quite a problem. While having both forward and back gestures is nice, I would much prefer an Exposé gesture.
My idea has been for a click with three fingers, although I'm sure that Apple has done plenty of usability tests regarding that and could come up with a slew of research to show me why that was a bad idea.
The major success story of this mouse is the removal of the scroll-ball. That scroll-ball has been the bane of my existence in the time I've used a (now-called) Apple mouse. I eventually had to retire one of my mice due to the scroll-ball continually jamming, even after a dismantling and cleaning.
The momentum is quite easy to get used to as long as you've used an iPhone and while I was slightly dubious about it at first, I've found myself not being able to turn it off.
Overall, the mouse is a nice feeling in the hand and I'd never want to go back to using an Apple Mouse. The only improvement would be a few more gestures.