What features won't I get with a Magic Mouse under Windows XP and 7?
I have a Magic Mouse on my laptop, and dual boot between OS X and Windows 7.

The basic mouse functionality works well, but the scrolling does not work at all (neither vertical or horizontal), and the gestures do not work either, as all of these functions are done by software and don't use the standard MouseWheel commands that windows expects.

I set it up a long time ago so my memory has faded, but I think you may also have to install the some apple mouse driver package from the Snow Leopard Boot Camp DVD too, but hopefully I'm wrong and you don't need this.