SInce I have personal experience I thoguht I'd share my $.02. For the first 20 years of my life I did not waer a helmet and fortunately didn't crash on my head. I even worked as a messenger for 2 of those years and got away free from injury. However, on night, for no reason at all, I decided to wear my helmet while riding to a friend's house. It was dark and while riding I got to know one of Boston's finest potholes personally, I flipped the bike and landed head first on the concrete. Fortunately, I was able to get out of the street and to safety before any cars came. My helmet was smashed in quite nicely and since that accident I've not ridden without one.

Cheers

Rob