Whether I've got my XC lid or full face on, I grab these pads. Initially, they chafed the back of my knees a tad, but the neoprene stretched where they needed to and now I don't feel a thing. I tend to forget to take them off when I go to get that post-ride burrito.
Last spill I took, they slid down a tad so I might have to tighten up the Velcro straps a bit but I've yet to have any kind of failure. They've protected me exactly as I've expected. When they finally fall apart, I'll be getting another pair or looking at something with d3o.