Having the right protection for mountain biking is a must-have! I definitely learned this the hard way after taking a rough fall last season (I have really gross photos to prove it). After that tumble, I immediately purchased knee guards so I can stop adding to my scar collection.
I ended up buying the Fox Racing Enduro D30 Knee Guards. These knee guards have been great for me so far! As expected, they kept my knees safe the next time I fell. They show no damage after my numerous falls since then. They are pretty lightweight and breathable, which keeps sweating to a minimum. Both myself and friend-of-the-shop Raphael have found the fit to be phenomenal. They stay right where you want them and don't slip at all. Do yourself a favor and get some knee pads before you suffer from ugly knees!