Although I haven't received my Magic Mouse (delivery was help up by an Apple Remote on the same order and I wasn't at home for it to be delivered today), I did have the opportunity to play around with a Magic Mouse in an Apple store last week.
To be honest, I was very disappointed. The mouse works pretty well on the whole. It has the same right click method that the Mighty Mouse has which means you can't do both a left and right click at the same time. That was to be expected. Scrolling up and down seems to work very well.
However, my big gripe with the mouse is that they've left so much of the potential unrealised. Its basically just a smooth mouse with an invisible scroll wheel. It could have been so much more. I was expecting them to utilise the multi touch surface in a similar way as on their laptops. I.e. pinch to zoom and pinch and rotate to rotate. There's absolutely no hardware reason why they couldn't have added this and I am truly baffled as to why they didn't.
So all in all, right now it really is nothing special. Better than the Mighty Mouse by a long shot (due to the lack of the easy-to-fail ball) but it offers no real advantages over a Microsoft or Logitech mouse. They killer feature should have been the multitouch surface but they left it largely unused.
Hopefully they'll remedy this with a simple software update to recognise different gestures but as it stands, the Magic Mouse is a serious let down.