I find knee/shin pads are more important quality wise than elbow pads (not talking full on back protectors or similar!). I have some really good quality knee/shin pads (can't remember the brand, bought them about 7 years ago, see comment below) but make do with some elbow pads that are just OK.
The reason I use pads is because one day I came off going across some wet roots, and both of my knees slammed into a rock at about 15mph decelerating to 0mph in a flash (which I know doesn't sound that fast, but trust me, swing a sledgehammer at your knees at 15mph and you'll know about it). I couldn't even cycle out of the woods and had to hobble out with my bike. Almost threw up with the pain at the time, I'll never forget it. After that I swore when conditions were iffy, I wanted have some protective pads. To be honest I don't wear them all the time, just when it might be a bit wet or icy.