Poor battery life? My Magic Mouse would last 3 weeks of constant use with NiMH batteries on average.

However, this was only the case after I applied the software update for my aluminum bluetooth keyboard. The first few weeks with the mouse would get me only a few days worth of use, at best.

Maybe there's another bluetooth device nearby that's causing your Magic Mouse to work too hard, and suck up too much juice?

(Note: I don't use my Magic Mouse anymore because I've since fallen in love with the Magic Trackpad, which has survived even the most precise work with Photoshop for mocking up pixel-accurate UIs.)