How long have you had it? I've used to use a mighty mouse on my old G5 iMac for 5 years about 8-10 hours per day and the scroll ball is still working just fine, but of course it needs a good cleaning after every 5 days or so.
On my first mighty mouse, the scroll stopped working after a few months (don't know the paper trick). Spilled juice facilitated a new mighty mouse & keyboard, this mouse has been going strong over 3 years now.
I originally purchased my iMac with wireless keyboard & mouse. They seemed designed primarily to keep the battery manufacturers in business. So I went to Apple Store and got the wired mighty mouse & aluminum keyboard w/ number pad built in and am very happy.
People who test drive my iMac give more compliments on the keyboard than anything else!