Best mouse ever is Logitech's old Mouseman Wheel. They haven't made them in years, but the current models are decently close. The MX-518 or G5 are probably the best gaming-wise. They also have an extra Exposé button on top, which is a great feature on any mouse that is going to be used on a Mac.
That MS mouse isn't bad either, and their Mac drivers are better than Logitech's - I just think that the thumb button is in an awkward position.