I'm agnostic on Glucosamine. From what I've seen, the studies have been mixed and some show no repair in knee cartilage, while at least one double-blind study did show statistically significant reduced pain.
I'm more interested in Chondriotin Sulfate and Hyaluronic Acid than Glucosamine. Chondriotin sulfate was studied in the 1970s as a heart disease cure and showed promising results in animal models and even in some human studies. Unfortunately, I don't think any quality follow-ups on this have been given. It doesn't seem to have any negative side-effects.
I've also had people swear they have gotten results from supplementing Knox gelatin. I've started supplementing gelatin and Chondriotin to help with an ankle injury. Too early to tell if it is effective, but it is inexpensive.