I'd hate to disagree with Gonzo on their effectiveness, because I have damaged my knee in the past, and used it as a support, and to keep the knee warm while skiing, and it has worked well.
I don't use it when my knee is OK. I think there was a discussion here a few months back about that.
The concern is that if you use the brace all the time, then your leg muscles, etc, will become weaker, not stronger, because the brace is doing the work for you.
In my case, I wear it when my knee is sore, but I don't wear it otherwise.
So far you've got an answer from a lawyer and a mechanical engineer/computer consultant, if you ask nicely, maybe some of our more medical/fitness professionals could add their thoughts...