I played with one at my local Apple store this afternoon and I left not liking it despite being really enthused about it prior.
While I found the scrolling and forward back gestures natural and intuitive, I found the mouse to be not 100% responsive. Sometimes a flick of my finger would initiate a nice smooth long scroll, while other times, the same motion would result in a couple of lines of movement of the screen. I found the amount of finger movement to screen scrolling to be very unpredictable which makes it pretty much useless.
If you are like me, you scroll up and down through forums and threads by scrolling the screen under the pointer... stopping when what you want click is under your mouse pointer... rather than moving the mouse pointer itself up and down on the screen... if this sounds like you then very predictable and fine control over scrolling is critical. This new magic mouse did not deliver that for me.
Another annoying design flaw is that the top surface of the mouse is a glossy plastic...like the white macbook enclosure. It's not very condusive to sliding a finger over... any amount of persperation causes your finger to stick and not drag smoothly... it's insane they did not go with a surface like the trackpads or even a brushed aluminum which seem to be immune from perspiration.
The other absolutely stupid thing about this mouse that I did not get use to in the short time I used it was that your mind quickly adapts to thinking it's a track pad... so you start wanting to tap the surface to click, not press the surface like a button. You also want to use two fingers to right click, which of course, doesn't work.
I think it's half baked. If you are a big track pad fan, you will be disappointed. If you are a big mouse fan, you will find the scrolling control unacceptable.
Just give me a damn track pad for my Mac Pro. I guess I'm going to try the Bamboo Touch. Maybe that's what I want.