I've used both. I prefer the MS BlueTooth mouse because it has a better feel and can be set for incredibly fast tracking. It seems like the Apple mouse caps out at about 50% of the tracking speed of the MS mouse. It just feels big and bulky, too. The only complaints I have about the MS mouse is that the scroll wheel is sometime difficult to use without unintentionally depressing the wheel, which invokes different functionality altogether. Also, I lose BlueTooth connectivity with my MacBook Pro every now and then, forcing me to either reset BlueTooth or reconnect with the Mouse, which is annoying and frustrating. I never had that problem with the MightyMouse. If you're accustomed to using a wired MightyMouse on other Mac systems, the size of the MS mouse might feel a bit odd to you. In addition, if you use a wired MightyMouse when you're "home" with your laptop and a BT mouse when you're on the road, if the BT mouse you're using is not an Apple MightyMouse, you'll probably have to adjust the tracking speed every time you switch mice.
M. McConnell