I was happy with my 20" iMac, but dissatisfied with the scroll ball, which I could never clean satisfactorily; it would work briefly and then fail. The touch surface on the Magic Mouse provides the functionality that the original mouse was designed to have, but wasn't reliable at. If you like scrolling, you might want the new mouse even if your Mac is otherwise fine.
There are a couple of hacks that add more multi-touch features to the Magic Mouse: Better Touch Tool, which I used for a while, and MagicPrefs, which I’m using now. I use a three-finger-click as a middle click, and three-finger up and down swipes to trigger Exposé.
The Magic Mouse hardware is capable of a lot more gestures than Apple officially supports. After using the hacks for a while, I understand why; it’s really easy to accidentally trigger the wrong thing. It’s a minor frustration for me, but novice users could get really put off if they expect to open a link normally and accidentally open it in a new tab (what I use middle-click for).