I used to wear one, and was even more convinced after my ex-Mrs, was in hospital for a few days with concussion, seemingly mitigated somewhat by her helmet.
Then I started reading up on it, particularly the Australian or Ontario experience where the rate of head injuries (per cyclist-mile) hadn't actually reduced at all, maybe worse if anything, despite helmet-wearing rates going from pretty low % to 95%+. WTF is going on ? Seems they don't, on average, help at all ! Of course, a helmet might help somewhat in some accidents, but conversely, going by real-world numbers, they must make other accidents worse. At the very least, by simple geometry they make your head nearly twice as big, nearly doubling the change of hitting your head in any given accident.
The other bizarre thing, is each time some kind of science is quoted to support a campaign, on examination, it is either simply rubbish, or misrepresented, or downright dishonest. If there was real numbers supporting the case, why publish lies instead ? Seems it's become a quasi-religous cause where any and all means are justified. People then get really angry if you try and put the unconvinced side of things
So, I no longer wear one.