Finally a knee pads that stay up
I have tried several brands of knee pads, my favorites until now have been the Kyle Strait 661's, but I always had problems with them sliding down and they are a bit bulky on the side.
The POC Join VPD 2.0 Knee pads have been great, 5 consecutive days of wearing them with tons of climbing miles and absolutely zero issues, no problems with them sliding or moving out of place. In fact I often forgot I was wearing them. I have tried the Fox, 661 and Troy Lees and by far the fit of the POC's is unbeatable.
They run a bit small, so I would say if you are on the fence about size, error with the next size up.