From my limited use of it: clicking with it--since the whole body clicks--moves the cursor out of place, such that I lose precision clicking. The scroll ball is atrocious. It gets dirty and loses function, and it's not easy to clean.

I goofed. I use a Magic Mouse, not a Mighty Mouse.

I work at a place with about 10-20 Macs. The batteries die faster than I can replace them. I get about 20 batteries per month and we churn through them. I gave up trying to get people to turn them off, put them all in a draw and a helpful colleague replaced all with el cheapo MS corded mice. Much better.

Any functionality that provides scrolling or zooming is completely whacked with it. Google Maps, for example.