As with anything, there are always people on both sides of the fence. My son is 13U this year, and has worn them for the past 2 years. After reading these posts, we have concluded that he is going to do his own trial. Wear them, and not wear them. We are going to video his receiving, blocking, throwing down, look at pop times, etc., with and without them. He is also going to determine what he thinks works best for him, considering comfort, fatigue, and performance. I really doubt they can do any damage to the knee (20 years of outpatient physical therapy/sports medicine experience, orthopedic certified specialist). I don't believe they are sturdy enough to make that happen. I can see them creating some lazy tendencies if used improperly. That is not an issue for my son. IMHO, the best way to know for your self is to do your own testing and see what works best for you. With proper conditioning, training, stretching, maintenance, and nipping physical problems in the butt early, a catcher should not have knee problems simply because they are catching.