I'd just like to add that I really can't use the mightymouse at all. Right-clicking is very unintuitive as you have to completely remove your left finger before it will register a right click. As I tend to leave my hand resting on all three buttons, this means moving two fingers instead of one.
Also, everyone I know who owns one has managed to gunk up the scrollball so that it only scrolls either up or down.
I'd recommend a sensibly-prised USB optical mouse with a scrollwheel. Also they tend to be about half the price of Apple's one.