I busted the meniscus in my knee a little while ago. I wasn't really able to walk for about 4 or 5 weeks afterwards. Since another hit would most likely put me under the knife to have it removed with the associated months out of action, I chose to get some Fox mtn bike knee pads (this was about 2 weeks after I could walk ok again...).
They are pretty bulky and I feel pretty silly in them (they are quite comfy however), but they should protect my knee enough to keep me out of surgery in case of a smaller hit. Especially prudent with early-morning ice about in Aus at the moment too.
I'll probably give up on them sooner or later, once my knee has mostly cleared up.
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?