My brother was in hospital once. 2 inch crack in his skull, blood clot and swollen brain, which pushed down on his optic nerves as it had nowhere else to swell to, causing severe pain and sight loss, major headache, we couldn't even wear shoes (trainers only) when we went to see him as everynoise severely hurt him. the only pain killers they could give him was paracetamol because they had to do observations on him (such as ensure his eyes dialated) every 15 mins.
He did this after falling over in a sports hall when playing football. When the ambulance arrived they, and his friends, told us he was talking gibberish. Anyone who knows anything about head injuries knows this is VERY bad (talking violently or gibberish can be worse than being unconscious as it signifies brain trauma/damage).
Knowing he did this simply by falling over, means i wear my helmet all the time (particularly as i once came off myself WITH my helmet and was unconscious for 1/2 hour and then had concussion for 3 months (dizzy spells) and occassionaly still get them.
I'm not suggesting they'll make a difference if you get hit by a car, but if you just happen to come of your bike and hit your head then, yes, i think they work.
But personally, i think if you don't wear a helmet... it's your choice... and your funeral. I'd rather not risk it myself. i quite like my personality as it is, although some may disagree