I have some but have not used them for a couple of years. The knee issues I was having have gone as my joints have got stronger.
There is little doubt though, that they take stress off your knees, particularly on descents. I also agree that only a few people use them properly. the strain should be taken by the strap, not by a white knuckle grip. My wife is using them a bit as she has had some knee aches and they help.
Personally I stopped using them as I don't see the need in the peaks/cheshire, just got out of the habit. I also like to have my hands free for taking pictures, checking GPS, blowing nose etc. It's a pain to have to keep dropping your ploes to do anything.
I must admit that if I ever get a knee twinge I start to wonder if |I should have brought them, been OK so far though. If I get issues again they will be out I'm sure.