I think you'll find most people use Mighty Mice because they are Apple and they match; I doubt anyone could honestly say it's a brilliant mouse.
For alternatives, as above, the Logitech I use is well regarded on here and on other Mac forums. Small, but not too small, tracks well, lasts ages on batteries, has an on/off switch and the Bluetooth connection doesn't drop out.
The only real downside to it is the fact it only has 3 buttons so I map expose to the centre button.
You might read that a lot of devices drop out with Bluetooth on OS X but generally this has been fixed some time ago so a lot of this is out of date.