At first when I got these, I thought they were awfully skinny to protect my knees. But they do a great job. I can fit them under my snowboard pants (I use them for snowboarding) and I barely notice them. Whenever I fall, they are right there. Only had them slip off one time after a long sidewards slide down the slopes, but that was well after the point of impact.
Haven't done anything that I think could have damaged my knees yet, but it's great being able to get up immediately from a spill.