The mighty mouse drives me nuts. The two major things are right clicking and the scroll ball.
I cannot get use to the way you have to use the mouse which is lifting your left finger before you right click. Otherwise, it is detected as a left click.
Also, I cannot get used to the scroll ball. At first it scrolled to slowly so I sped it up and it worked fine. But then it was too sensitive in UT and other games that require single ticks on the scroll wheel.
So I bought the Logitech S530. I will try it out this weekend.