I just bought one after starting to do some local racing, and was glad I did. The first use for it was when I was pushed up high on a quarterpipe riding boardercross. As I came off of the quarterpipe my body was angled horizontally to the slope (as I flew through the air). All I could think, as the flight was in slow motion, was how much this was going to hurt. When I did impact, and my head snapped back, I was surprised that it didn't hurt at all. That was worth buying the helmet for.
I wiped out recently this year; not sure what happened, but the board came out from me, and I dropped backwards; head took an almighty slam. It actually hurt - and that was with the helmet on.
I took a slalom gate to the face last weekend. Mmmn fun. That's why I use a full-face for racing now (damn ski-racing gates). Nothing quite like taking a gate to the face and being able to keep on riding.