I've tried it once as well. I did quite well I think and it was all good fun.
I remember one time I got hit though. I was sneaking behind my brother (he was on the other team and in some kind of fort) and when I could, I took a shot at him some 10 meters away. I got him but I hadn't noticed his friend.
He was sitting at a lower angle and when he started firing at me, I instinctively turned around. Unfortunately, he managed to hit my jaw below the mask. It started bleeding slightly and I got a big, red mark. Still, it was good fun!