You could call it a Powermat for your Magic Mouse, because this is a wireless inductive charger--you swap out the standard batteries for the Mobee rechargeable battery, then lay your mouse on the base station (which has to be plugged into a USB port) to charge the battery.
Mobee Technology says you'll get about six days of "autonomous power" before having to lay it back down on the base station (it takes about 6 hours to fully charge so you'll probably want to do it overnight). The rechargeable battery is rated at 500 cycles, which means you can charge it at least 500 times before it doesn't hold a charge anymore. In theory, it should work for several years.
If you consider how many times a year you have to change out your mouse's batteries, it would probably take a few years for your investment in the Magic Charger to pay off. But not chucking those batteries in the garbage makes you a more environmentally friendly person. Right?