I picked one up at the NYC SoHo store, they had just opened the box from the shipment that day. I got the first one, and they were sold out by the time a left the store. That particular store, however, is relentlessly busy at nearly all times.
Anyways, t's a nice mouse, feels ok and has a nice heft. I personally wish it was a tad wider. I works flawlessly and looks great. No middle button/expose button does suck, so I reset my hot corners to take care of this until there is a 3rd party solution.
I do wish I had the option to use tap-to-click on the track pad/top shell on the mouse. I've become accustomed to thos on my macbook pro, and it would be nice if I had the option to lock the physical click and just use tap.
I really like that the mouse is symmetrical. I use a mouse with both my left and right hands and the design of the Magic Mouse makes it easy to switch.
It's not perfect, but in time and with the right drivers this will be a powerful device.