So far I'm pretty happy with it. My biggest complaint about the previous Apple Mighty Mouse was that the right click was really erratic. Not a big deal on OS X since it doesn't use right click too much, but a major issue when running Windows (under Parallels). Right click on the Magic Mouse seems much better. The other less serious, problem with the Mighty Mouse was that the scroll ball would get dirty and quit working, but it became second nature to clean it (by running it around on your hand or pants).
My only complaint is with the left and right swiping for horizontal scrolling - it's a little too easy to trigger by mistake. Again, that doesn't matter most of the time because there's nothing to horizontally scroll. But in Adobe Lightroom I keep switching images by mistake. And it's touchy enough that it zips through half a dozen images, making it a nuisance to get back to the one you were working on.
I tried installing Better Touch, a free program that lets you tweak all kinds of Magic Mouse and touchpad stuff. But for some reason it doesn't seem to work with Lightroom. Maybe Lightroom is using a different interface to the mouse. There doesn't seem to be any way to adjust the behavior in Lightroom either. I'd be happy just to turn off horizontal scrolling by left and right swiping - it's not something I need a lot.
I'm still getting used to the scrolling with momentum. It's a bit like the iPhone/Touch. I've been catching up in Google Reader and it seems to work well there.
Another minor issue was the lack of a middle button. On the Mighty Mouse you could click on the scroll ball to get a middle click. I had this configured to trigger Expose since I had the function keys set to be function keys (for Suneido on Windows on Parallels). Better Touch solves this since it lets you add a middle click.
