I find that they induce extreme cramp in my left hand.
I'm happiest with a huge optical mouse. I've currently got a v1 Intellimouse Explorer which is much better than anything I've had before. I don't get on particularly well with alternative input devices (the fact that most trackballs are sinistromanualist in the extreme doesn't help, and my big paws don't work too well on trackpads) and have come to prefer a wired rodent.
One thing I've noticed this past few years is that mice don't break; instead, they become so unhygienic that one replaces them in order to avoid certain disease.
With this in mind, I have an eye on the Microsoft Wireless Intellimouse Explorer. I shall go and have a play with one, but I think this will be The One.
[Microsoft make nice mice, somewhat less nice keyboards (although I haven't played with the latest crop of wireless ones), reasonable games and good Mac OS X software. Now, if we could only get them to drop this silly "Windows" idea they'd have a product range everyone could agree with.]