I am skeptical about the claims that the glucosamine/chondrotin supplements actually slow deterioration of cartilage, or promote repair. If it were truly proven via scientific studies, there would be a medical protocol for it and - given the fact that there are almost no side effects - it would be the first line of treatment in arthritis management. When I have looked into it, I haven't seen any claims of cellular repair, slowed damage, or any other purported physical effects that weren't vaguely referenced and part of advertisements.
Additionally, every anecdotal report and testimonial I've ever heard stated that the perceived benefits of the supplement/medication quickly disappeared as soon as one stopped taking them, which is not consistent with the idea that the stuff was actually implementing mechanical repair. Hence, while there is much speculation about these effects, in practice it seems to be little more than a pain reliever.
If you know of specific studies which show otherwise, please reference them. I dug around for a while via net, and couldn't find such information - especially nothing that used observational methods like biopsies or scans to demonstrate actual cartilage repair or slowed degeneration.