I originally got a Magic Mouse with my 27" iMac.
I have to say, I've always liked heavier mouses. It clicks. It has "inertial" scrolling. It right clicks. It has 1 finger 2 directional scrolling.
I like it. It's kinda like a cross between a trackpad and a mouse. I use it and my MacBook Pro on a daily basis. As a comparison, I use the Revolution MX, Performance Mouse MX, and an old Microsoft Optical Explorer Trackball for any Linux or Windows box I own. It is a little low and has a very specific position it's actually kinda comfortable. And there's 3rd party apps to add gestures.
I use it with Steam for Mac. Portal is acceptable and some of the slower paced indie games are fun to use with it, but I'll stick with my Performance MX for gaming.
My friend and I were thinking though. It would be much more useful if you rotated it sideways, had it click to the left and the right, and had a more ergonomic handle sticking out of it, so it looked more like a T. Then with 4 fingered gestures and everything, it'd be awesome. But that's just a little tangent.