When I ride road or MTB I always wear my helmet, in fact I feel rather naked without it. But then I started commuting (in this I'm including my rides to and from work, rides to the store, to visit friends, to dinner, etc.) and it was easy to just hop on my bike with no gear and go. It makes you feel like a kid - kind of free. I figured as long as I was going slow, paying attention, or on trails I was OK. I'm still not sure I disagree with this...
But a couple things happened to make me rethink whether I wear a lid or not:
First, I crashed just playing around in the park when my pedal came off (stripped thread). Hit my noggin hard enough to break my helmet and give me a slight concussion and lot of neck pain. I took it at face value - I was playing around, these things happen.
Second, I started a group ride to get beginners into cycling. I found I wasn't worried about myself because I have ridden enough to be comfortable with my abilities (aside from freak mechanical issues). I was worried about the newbies - they scared the crap out of me and I realized I needed to set an example.
Not to mention that I felt the need to set an example for all the kids out there. It's one thing for an adult to make a decision about not wearing a helmet because we can comprehend the consequences - kids can't (and shouldn't).
Anyway, I am now a commuting, helmet wearing member of the community. In order to help entice me to wear it I bought a fun new one from NutCase Helmets:
However, one of these days when I have lots of extra cash I might try rocking one of these Yakkay numbers:
They are available on The Bicycle Muse website.