Cool head, dead head?
I commute by bicycle. Along my route, I see a large proportion (though not the majority) of other cyclists who do not wear helmets, even though it is the law to do so here in British Columbia. Though the law is never enforced (that I've seen), it reflects the smart thing to do. Even before it was the law, I wore my helmet every time I rode.
Particularly infuriating are those riders who have helmets, but leave them slung over their handlebars as they ride. I know personally that head-bonking bicycle accidents don't happen only on busy streets, or at high speeds, or on days when it's not so hot that helmets are a bit uncomfortable. I've seen people fall over, in ways that could easily have cracked their skulls, while getting on their bicycles.
The closest I came to such an incident was while I was mountain biking on the trails of Mt. Seymour in North Vancouver a few years ago. I was descending a particularly steep chunk of trail (very slowly, I would add) when I did an "endo" -- I flipped over the front of my handlebars and conked the top of my head right into the thick bark of a Douglas fir tree. I was fine, but without the helmet, and despite my low speed, I would almost certainly have been knocked unconscious.
So if you ride, wear your helmet. Every time, all the time. Sure, it looks a bit goofy, but it's better than being dead.