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.