I wear one nearly all the time...I used to only wear one when MTB'ng and it took me a couple of concussions before I began wearing one off road. It's funny I live in a state with no helmet law for motorcycles and roughly 98% of the cyclists I see wear helmets.

The key to me on the helmet thing was a lot of trial and error. I tried numerous helmets over the years and I will say they have come a long way from the Lycra covered styrofoam cookers that I hated to wear. Now I still hate the thing at times in the summer...but since I have begun using a helmet I have never had a crash(knock wood)...whilst commuting anyway...offroad that's another story.

I think any sensible bike commuter would and should wear a helmet...it's common sense...but to each their own...I do ride a fixie every-so-often without a brake so who am I to say.