Apple had not been that happy with mouses unfortunately.
The Mighty Mouse was awful. The cord version was light and easy to use, while the wireless version was too heavy and clumsy due to the batteries. The trackball was the WORST thing on earth. With so many people complaining about it and the problem is that, its mechanics inside just get dirty if you sweat in your fingers and then it stops working. I had to open the damn thing to clean it once every two weeks or less.
Along came the Magic Mouse. Up to Snow Leopard, it suffers from the same sluggish pointer movement, being fixed in Lion. However, as it still uses two AA batteries, it is way too heavy and clumsy, I still vote that they do need to make these with AAA batteries. Who cares if you need to change it more often? At least you can use the thing everyday without feeling you're dragging a stone around.
Then there is the multi-touch gestures on Magic Mouse. Although nice, there is a good learning curve from Snow Leopard to Lion. Before, double fingers to go back, now it just changes the desktops, bla bla bla. Further more you find that some options are not available for it. If we have two fingers left / right / click why can't we have two fingers up / down for LaunchPad and Desktop?
Finally, with the trackpad all those issues go away, you have full multi-touch options, rotate, zoom, all the fun stuff, without the clumsy mouse.
Granted, if Magic Mouse was lighter I think I would still keep it, but being so heavy with the batteries it kind of misses the point to me. My next iMac is definitely going to be with a trackpad.