To all those who have said "without my helmet I would have died..." or words to that effect, I would suggest that death would be the best outcome. More than one study of motorcycle helmet effectiveness (and I apologise for not having references handy...) has shown that in fact the helmets don't do much to stop people from being killed. If the impact is enough to kill you without a helmet, there's a bloody good change it will still kill you with a helmet. It's the impacts that don't cause death where helmets help - eg. instead of having bad (and most likely permanent) head injuries, you may just get a concussion.
Sure, bicycle impacts are likely to be at a lower speed, but bicycle helmets are pretty ordinary compared to motorcycle helmets.
I have a friend who got some nasty head injuries from a knob in a car who hit him. He was a careful, competent bicycle rider and yet he still got hit. He was wearing his helmet. He still got a fractured skull, even though it was obvious on inspection that the helmet absorbed some of the impact. By 'nasty' I mean permanent - he is not the same person he was. Would he have died without the helmet? I don't know, but I don't think so. At the very least his 'nasty' injuries would have been worse.
If you want to put the fate of your head in the hands of all the incompetent drivers on the road, that's your choice. A helmet may not save your life, but there's no doubt it will reduce the head injuries that you will get when you some knob in a car cleans you up.
Even if it just saves you from loosing some skin, I think it's worth it.