Yeah I think they are quite useful for snowboarding firn too, but how inconvenient are they? say 100points for maneuverability in normal conditions and how many points with the pads on your knees?
They do mess with flexibility a bit I guess, but to be honest I didn't notice them over the stiffness/pain from a busted knee :P
On my good leg, I guess no worse than about 75%... depends how low you want to get. I mean, they are designed for use on a bike, so they are pretty flexible, just really fat.
The ones I got have a hard outer shell/bubble about an inch out from the knee, with a big fabric-backed air-gap behind that, so there's room for the knee to move. The main constriction actually came from the straps, which bunched up and pinched a bit when bending really low.
I guess it makes it a bit harder to actually bend the knee too (like bracing it or something), but that was good for me in the circumstances. Have to remember that I had two big compression bandages on under the guard too.
by the way, for anyone who's interested)