I tried the Magic Mouse a couple of times in the store before buying one to use at work alongside my MBP. I'm quite picky about mice and have been using a SteelSeries Ikari Laser both at home and at work but find that I never use the side buttons, except accidentally.
What sold me on the Magic Mouse was kinetic scrolling, I can't imagine how I lived without it. It's also so sleek that it slips discretely into an elastic pocket in my laptop bag. And, finally, it gets a lot of attention at work.
Ikari Lasers seem to be about £50 a pop, which makes the £55 for such a stunning wireless mouse from Apple seem entirely justified. That said, the pretty good Microsoft Notebook Mouse 5000 can be had for under £20.
The one thing that irks me at the moment is that the right click area is a little bit small, and I often find myself struggling to right click but the time I save zipping through reams of code with kintetic scrolling more than makes up for that.
It also works with my Netbook hackintosh, which is nice because the touchpad on the Samsung NC10 is a disaster.