I have severe chondramalacia patella & crepetis in both knees, plus a minor tear in my left ACL. Been taking Glucosamine supplements on & off for the past 10 or 15 years. While I'm taking it I never feel like it's really making much difference, but after I stop taking it for ~2 or 3 weeks I start to notice the aching in my knees more, so that generally triggers the on & off cycle to resume.

I'm perfectly willing to believe it's a placebo.