I bought a pair of Vibram Five Fingers -- as long as you run on the front of your foot (which takes a lot of getting used to) they're great. I'm a big, overweight guy, and running has always meant a lot of pain in my knees, shins, feet, ankles, back, etc. When I switched to the Vibrams I had to deal with my calves and feet being extremely sore (the muscles doing the work) but all other pains disappeared.
I'll take sore muscles (which went away over time) over bone/knee/cartilidge damage any day of the week.
I need to find something like the Vibrams for wearing at the office.