I was out riding with a mate of mine on dirt bikes a few years back, he always wore a full face even on the dirt, anyway a rock flicked up off my tyre about the size of a golf ball and hit him square in the visor it shatered the visor but he didn't recieve any injuries from it, it is scary to think what would have happened if he had been wearing an open motocross style helmet, a few weeks later we were out riding again, and he hit a tree hard when we were going down a tight track through the scrub, the bike completely missed he caught it with the top of his body on the inside of a corner, he was pretty beat up, but managed to ride back out to the cars, there was basically nothing left of the chin bar of his helmet, he had numerous broken ribs, a broken shoulder and collar bone, punctured lung and a broken nose. He was bloody lucky about 6" above where his head hit the tree there was a metal spike hanging out of the tree about 10"long, I loaded the bikes up onto the trailer and rushed him to the hospital, all the while with him telling me he didn't need to go to the hospital, he just needed to go home and put an icepack on his nose, anyway after a week in intensive care, another 2 weeks in hospital he was released to go home but it took him another 3 months to get back to work. If he hadn't been wearing his helmet I would have been bringing his body back from out there that day, that I'm sure of it. When I saw him fold around the tree I was almost certain that I would be telling his missus and kids that he wouldn't be coming home, the couple of minutes he was unconscious were the longest few minutes of my life. But thankfully he was wearing a helmet.