I love my BT mighty mouse. Feels good in my hand, love the little "nipple" -- works really quick and smoothly. Have seen posts that indicate they get dirty quickly and need frequent cleaning, but in 3 months of use, I've not had any problems. Biggest "problem" is that when I take the MB from my desk into the living room (two walls, about 20 feet), the MB still recoginzes the MM. I always forget this, and have to go back and get it or go back and turn it off to use the trackpad! Not much of a "problem", I'd say -- great range!
The mighty mouse (corded) that came with my iMac works fine, but I would not buy one. It's very difficult to right-click consistently. I don't use the sideways scrolling feature on the scroll ball very often and it's also difficult to consistently scroll sideways. Occasionally the scroll ball jams and it's a pain to clean. A regular mouse with two real buttons and a vertical scroll wheel would be much better.
The scroll ball on the mighty mouse is so vulnerable to jamming due to dirt and grime that I would look elsewhere if I was planning on buying a portable mouse.
I like my MM quite a bit. An improvement over the original Apple Mouse.
That said, I'd consider alternatives such as Kensington or Logitech. A lot of people like the Logitechs and I liked the Kensington mouse when I saw it. The nub on the MM has me frustrated(on a design level) and looking around.
MM is really overrated, and overpriced for what you get IMO. The Logitech mice, for one, are much superior. MX1000, G7, etc. are all great mice, which I like much more than the MM.