I bought a Magic Mouse last week and absolutely love it. It has good heft, perfect weight distribution, and meaningful tactile and audible feedback; it lacks the side buttons of the Mighty Mouse, which drove me nuts, and there’s no longer a scroll ball to constantly clean.
It brings the useful and elegant scrolling with momentum feature of the iPhone OS to Mac OS X, and the execution is spot-on. Gestures capability just puts it over the top, though I find it difficult to two finger swipe to the right. I particularly love the swiping feature to sift through days in iCal, go back and forward in Safari, and move forward and backward through my inbox.
Scrolling with momentum is available for most all of us running 10.4 or higher, even without a Magic Mouse. I found a utility called Smart Scroll that enables this cool feature and many many others that you’ll soon find it difficult to live without. The software has a fully functional evaluation mode, and is $19 if you choose to keep it.