Well I've always worn a helmet and also believe that they are of benefit, even if there is no solid proof either way.
I now am even more convinced that they are worth their weight in gold for safety, as I unfortunately found out today that they work, I was hit by a car that the driver decided that as he was coming towards me, I was almost level with his wing he would try and turn right, all I remember is the initial oh shoot and seeing his headlight/bumper hit my front wheel. Eyewitnesses accounts are that he hit me, I went up and over the bonnet and came down on my head, my helmet a specialized D2 broke in 2 places, but I only suffered mild concussion and a few minor cuts and bruises, the bike is bust up and probably going to need at least a frameset and front wheel (his insurers are going to love this as Scott Addicts and Zipps don't come cheap, let alone and of the Sram Red or Ritchey that maybe damaged too).
So for me landing head first after being hit, the doctors, paramedic's and police all said my helmet saved me big time, oh and the Garmin said I was doing 27.2 when I was hit, so helmets work at proper speeds too.
Time to go and buy a new one I think and send the bill to his insurers