I'm with a2psyklnut. Hit head-on in 1994 (by a bicyclist!) and sustained a concussion DESPITE the helmet. I was doing nothing wrong, going all of ~8 miles per hour up a hill, and there was nothing I could have done to prevent it.

The concussion took a while to set in, so the guy who hit me had already left when I went to pieces (so my friend Chad tells me, I have no memory of it). Chad went to find a phone and call my parents, and some other cyclists found me lying in the grass with my crumpled-up bike, and covered me up with their jerseys in the meantime. The first thing I remember clearly is being put into a neck brace by an EMT an hour later, and rolled onto a body board. What I'm driving at here is that some things happen so fast you don't have any chance to react.

Hit again last year by a car turning left as it came towards me. Nasty grind marks on my face (but no teeth lost, thank goodness), a crunch to the left temple of the helmet since I was thrown face-first onto the pavement... again, I was doing nothing wrong and there was nothing I could have done.

I can't see any reason to not wear a helmet. They're not expensive (US$30 for a light, well-ventilated Bell). Besides, I need something to attach my 3rd Eye mirror to.