I've always worn a helmet on a motorcycle, but never on a bicycle. Just what I'm used to, I suppose.

The BBC article summed it all up very well:-

1) On a statistical level, injuries are increased by helmets, due to risk compensation*;
2) Helmet laws have been proven to cause people to give up cycling, so their net effect on health is negative; which is probably largely because
3) For some reason, no-one has ever designed a non-dorky bicycle helmet**; and yet
4) On an individual level, they can still save your life and reduce the chance of serious brain injury.

*I personally am more cautious on a motorcycle than in a car, and still more cautious on a bicycle than on a motorcycle.

**Perhaps they would be cool if they looked more like motorcycle helmets, instead of looking like you had an inverted canoe on top of your head?