Though I don't use full face helmets myself on XC trails, something happened this weekend that made me reconsider. One of the guys I was riding with fell off the side of a trail (going uphill no less!) and rolled down the hill about 15-20 feet. A rock dislodged as he tumbled and hit him in the lower lip. He lost 4 teeth (a bridge) and had to get stitches to sew up his lower lip.