I've recently given up my helmet after a lot of research and thought. I feel that the cons outweigh the pro's.
I have kept my helmet, me and the OH are taking up mountain biking in the spring, so I think I'll start off with a helmet, as I guess I'm more likely to come off and bonk my head.
Funnily enough, I got told off by the coach at my sons cycle speedway training this morning for arriving without a helmet - said it was his official duty to do so as he was on official coaching buisness. Looked a bit gob smacked when I started to explain my rationale!