My reckless friend went out at lunch time the other day and didn't come back. Basically a pedestrian walked right out in front of him and they were both knocked out and taken to hospital (in separate ambulances). The joke is that he always wears a helmet to and from work but went out at lunch without it. Oddly he is not a reckless person but maybe he forgot he wasn't wearing a helmet.
So of all the cyclists I know at work - about 5. Two non-helmet wearers have be involved in a head injury type accident. One spent a month in hospital having been left at the side of road - the only clue being a knocked off pedal (mind you he is registered blind and wears hearphones - no joke).
I still have no "helmet wearing saved my life" stories from work so I say if you want to wear a helmet, do and if you don't, don't.