I have been taking glucosamine for nearly two years now. I originally started when I started upping my mileage on the street and started feeling twinges in my knees.
Whether this stuff actually cushions joints is up for debate. Regardless, it took a good few months before I "thought" it was working. Perhaps it's the good ol' placebo effect.
Since taking these, I've never had that "I'm going to break" feeling... in my knees, that is. In general, however, I feel broken because I'm tired and overworked (I had to call in "sick" to one of my jobs today because I was tired, fed up, wanted to go to the gym and shop, and had to work on my thesis and study for exams). But anyway, the knees are fine. I'm not sure if this is because I got stronger overall, but I'll take what I can get.
Currently, I am taking generic glucosamine, but I do switch off with Osteo Bi-Flex (Triple Strength with Joint Shield). It is not cheap (can be up to ~$30 an economy-sized/180 tablet bottle, but I have found them on sale for about $12). Their website offers coupons and a rewards program (I've been saving my "points").
The pills themselves are a little big and taste "muy malo" (i.e., bad), so chug 'em down quickly. I have mastered the art of swallowing the two daily pills and a multivitamin simultaneously.