I highly recommend
"MouseWizard" as an alternative add-on for the Magic Mouse if you're using a PowerPC Mac (on Leopard, in my case).
I've heard BetterTouchTool is good, but it requires an Intel Mac. The Developer at Samuco.net has been in touch with me through emails to continue to work on improving MouseWizard (w/Magic Mouse) for all systems. It has a basic, but nice GUI window for assigning gestures and further customizing the Magic Mouse's potential, which you can easily access from the menubar icon. Check it out, as this developer is very helpful and actually uses your feedback.
I mainly use MouseWizard for basics like Expose' and Spaces, but you can also assign gestures like 3 and 4 finger click/tap, middle click/tap to almost any process, app, or key-command. It also allows you to zoom Safari text/pages and even put your computer to sleep. I can now finally appreciate Magic Mouse's versatility. Thanks to Samuco...keep up the good work. It is $2.50

after trial period, but worth it for us PPC users that can't upgrade to Snow Leopard. (Strongly endorse)
Oh, and trust me...if the incompetence of the Mighty Mouse scroll ball annoyed the hell out of you like it did me, then you'll reeeally like the Magic Mouse. It's smooth like butter.

So far so good anyway. But that's just me.
Hope this helps.