I have a confession: I rode my bike without a helmet today.
Now, before anyone starts making the usual "organ donor" comments, I would point out that in light of the 80-degree weather (after weeks of 40 degree weather, soon enough 80 will be a cool day) I decided to take off my lid and drop it in my basket for PART of my ride, which was along the D&R towpath, a flat, smooth trail with no motorized traffic at all. When I got to the point where I had to get back on the road, the helmet went back on.
For non-cyclists, this might not seem like a big deal at all, but in the cycling world there seems to be a culture war between the helmet and non-helmet crowds. There is all sorts of sanctimonious blather on both sides of the issue, along with "proof" that helmets will either save your life, guarantee you'll get hit, make injuries worse, mitigate injuries or have no affect whatsoever.
As for myself, my position on helmets is that no one over 18 should be required by law to wear one, but since they do seem to offer some protection from head injury in a crash, it's not a bad idea. One should definitely wear one while racing, and when mountain biking, it's probably a good idea when riding fast or in traffic, and probably not all that necessary when cruising around back streets or on going slow on a bike path (but if you're used to wearing one, go for it). During icy or wet weather, you might consider one as slippery conditions make a fall a bit more likely.
Kids should wear them, because low-speed faceplants seem to happen for them on a fairly regular basis, and it's probably a good idea to wear one when riding WITH children, because you'll set a good example, and besides which when they suddenly decide to pull a Jackie Chan impersonation, you know it's going to happen six inches in front of you, so you might as well be ready for it.
I've worked in the bike business a while, and known a few people who've crashed hard and got off pretty easily in part because they were wearing a helmet, all of them on either off-road or high speed road rides, and I've had some minor dings myself (including the time I was hit in the head by my own bicycle) so for that type of riding, I think they're a good idea.
I guess that puts me on the pro-helmet side of the debate. On the other hand, if having to wear a helmet is the only thing keeping you off a bike, ride without one, you'll probably be fine. I'm also pretty sure that riding in a safe, predictable manner, and paying attention to your surroundings is much more important to being a safe rider (take, for example, the fellow who nearly hit me while I was locking up my bike today in Highland Park, he came from behind me, riding the wrong way on a sidewalk, and didn't make any noise or give any indication that he was about to pass a pedestrian, BUT HE WAS WEARING A HELMET). In the Netherlands, famous for being the safest and most bike-friendly place on earth, helmets are a rarity, but so are bike fatalities, so it has a lot to do with safe riding and and safe interactions between bikes, cars and pedestrians. So I guess I agree with the anti-helmet folks on some things too.
The thing that gets on my nerves, though, is how every time the topic comes up, or even when it's not specifically brought up (for example whenever there's a blog post or ad featuring a non-helmeted person) how there's this immediate, knee-jerk reaction of "he's an idiot, he's not wearing a helmet" which is immediately followed by a "helmets don't really make you any safer" comment, and then a general outpouring of obnoxiousness from the peanut gallery. It's stupid and I have the feeling it makes newcomers somewhat uncomfortable. If you're thinking "hey, I want to ride a bike more and get involved in some of these online discussion groups" but find out that before you can type three words you're expected to pick sides in a stupidly vitriolic debate, it can be somewhat off-putting, and you might find yourself thinking you need a better hobby.
I think helmets are good, and wear one on most (but not all) rides. I recommend them to people who ask my opinion, but in fairly mild language. If you don't want to wear one, that's OK too. Just have fun and be safe.
However, if the only reason you won't wear a helmet is because they're not cool or stylish enough, I have two words for you: bicycle fedora.