Gaming with the magic mouse would come down to being a personal preference. I'm not a hard core gamer, and for me it did not work out for the following reasons:
Size/Shape - It's not the most ergo-friendly mouse and I have above-average size hands. IMO it's not a comfortable mouse to constantly hold for gaming sessions.
Tracking/Sensitivity - I don't play FPS games much. Mostly RPGs and sometimes a MMO. The Magic Mouse doesn't have a high DPI and feels kind of sluggish when trying to move with it.
Lack of buttons - I don't need one of those crazy MMO mice, but I do like to have a few extra buttons for binding of other actions. Most common for me is binding a mouse button for PTT in Vent.
Personally, I like Razer mice. It's not because of the look or the brand, but the fact that Razer supports the Mac on all its mice, or at least all the ones released in the last several years. Logitech used to be my choice when I used a PC, but they don't support the Mac on ANY of their gaming mice, so I dumped them.
In the end only you can make the decision on what works. IMO the Magic Mouse is a great mouse, but I just don't think it was designed to be held in your hand for long periods of time. Which is something you would do while gaming.