I know a lot of people don't like Apple's Mighty Mouse, but I actually like the thing--at least the bluetooth version. It's small, fits in my backpack and pairs with my MacBook Pro consistently (which can't be said of all the Bluetooth mice I've owned).
The one I keep in my office, however had a problem: the scroll ball stopped scrolling up. Down, right and left all worked. It was annoying. I was wondering if I needed to send it in to be fixed (or simply buy a new one).
I fixed it. Turns out the sensor for scrolling up is particularly susceptible to dirt and dust. I took a wipe for cleaning screens and rolled it on the scroll ball vigorously for a minute of so and the problem went away.