I've only just seen this topic on my RSS feed so please excuse my lateness in providing advice...
From the age of about 12 to 17 I had various problems with my knees, mainly something similar to osgood schlatter's (it was never explained fully what it was, other than "similar") along with just generally weak muscles in the region. I started bocking at 17, which was 2 and a half years ago. Since then, my knees haven't exactly improved in terms of pain or annoyance, but they certainly have not got any worse and they are definately a lot stronger than they were. The worst I've done is a few hyperextensions, but my legs are used to that anyway just from walking and running on foot, let alone bocking.
The fact is, I know my legs are pretty crap. I was put on physio for 5 years and it never helped one bit, and was never changed when I stated it wasn't helping. Since I started bocking, I essentially gave up the physio but bocking has not made my knees any worse. It's high impact, it's obvious there's going to be the occasional accident involving your knees because of the way you wear bocks, but it's no worse than any other high impact sport, and the shock from bottoming out mainly goes to your back, not your knees.