Cover your melon
Just north of where I live, a 15 year old girl on a bicycle was hit by a truck yesterday. She's fine, and according to the investigator on the case, "...the helmet the girl was wearing was crushed."
I have personally cracked a helmet in half in a crash on the Golden Gate Bridge, and walked away from it (albeit with 2 years of back injury recovery). I can only be left with the assumption my head would have been the thing that cracked in half had I not worn a helmet.
I see so many roadie cool guys in both Chico and the Bay Area blasting by without helmets, and while I'm inclined to get on my soapbox about it, I guess I should take the Darwinian approach and assume they are more willing to be removed from the gene pool than I.
I know helmets aren't 100% guaranteed to protect your noggin - just look at Saul Raisin's accident (and I'm so happy that he's getting better). But you'll never catch me without a helmet.