Despite a lot of negative opinion, I got a wireless Mighty Mouse and love it. The scroll ball did have problems after 8 months of use, but I took it to the Apple store and they replaced it almost instantly without any hassle at all. It's covered under my Apple Care plan for another 2+ years, which is more than I can say for any other mouse.
After using the little scroll ball, I like it a lot more than traditional scroll wheels. The low profile design is great for tossing it in my book bag without a lot of bulge, and the bluetooth connectivity connects quickly and means I don't have to use a USB adapter. One of the biggest things is the squeeze Expose function I couldn't live without now that I have it.