I used to wear a helmet whenever i cycled as a matter of course, never paid it any thought one way or another. One summer a couple of years ago, on one of those rare 30 degreesC+ days, I was riding home from work and my head started to feel really unpleasantly hot, to the extent I felt a bit dizzy. It occurred to me that I would be safer if my concentration wasn't compromised by overheating. So I took the helmet off, and the next day I didn't wear it. I noticed a couple of things: firstly the feeling of the wind in my hair was pretty wonderful, and secondly I felt more vulnerable, which changed the way I rode - for the safer (fairly obviously). Anyway, the helmet went back on in the winter and I thought no more about it. Then the next summer I thought about going helmet-less. I left it at home one day and again I had the feeling that I was riding differently, riding better. I did some more reading about it and with the help of forum threads like this one, I learned a great deal. Now I don't ever wear a helmet and I am certain that I am a much better and safer cyclist for it. So, thanks to you all for feeling strongly enough to keep posting here, its helped at least one person make a better choice.
The irony is that by choosing to not wear a helmet, I am certain that I have spent, and do spend, more time thinking about safety that the vast majority of helmet wearers.