Oh Christ, another helmet thread. Were anybody to bother reading the numerous other helmet threads, they would already know that:
1. Where good figures exist (mainly from Australia and New Zealand) they indicate that increased helmet use has NOT correlated with fewer head injuries to cyclists;
2. The incidence of head injury to unhelmeted cyclists is very low - roughly the same per mile travelled as for pedestrians - so why don't we keep warning pedestrians to wear helmets?
...and many other facts that call into question whether helmets, for just riding around, are remotely necessary. As for the endless "I'd have been killed but for my helmet" stories, I'm sure they're sincere but the fact is you have no idea what would or would not have happened without the helmet. I have had the experience of crashing into a motor vehicle, and hitting it forehead-first, at about 15-20 mph. I wasn't wearing a helmet and I was unscathed. Had I been wearing a helmet, I can well imagine that I'd be attributing my survival to that helmet. But I wasn't. Was I lucky? Maybe. But skulls work pretty well, most of the time - and for much more of the time, they don't have to. Cycling is a very safe activity.