I wasn't wearing a helmet on my commute to work, for the reasons many use: heat, not on the street much, etc. But I signed up for a charity ride, which required helmets, and since mine was really old with poor venting, I bought a new helmet the day before the ride. At mile 13 of the metric century, I took a turn too fast trying to gain ground on a group and didn't hold the turn. I hit a curb sideways, and was catapulted into a different zip code than my bike, into rocks and bushes. Woke up with someone looking down at me asking me if I was alright, to which I replied "I don't know". The helmet had to be replaced, and there were bruises where it had been slammed against my head. CT scan was normal, which means I must have sustained significant brain damage. I wear a helmet all the time now, but hate helmet laws as it is a personal choice.

With a decent well-fit helmet, it's easy to forget you have it on.