I "always" wear a helmet: exceptions being a) when I forget to take it and am riding to the train station for work (3 miles) and can't be bothered to go back to get it, and b) when I'm in a generally non-traffic, high-visibility, flat situation e.g. a tennis court, park, basement parking garage, etc.

I've twice been grateful to have my helmet. Once in a fall this year over a wet painted strip, and once many years ago when attacked by magpies. Both resulted in helmet damage but no head damage. I've gone over the handlebars twice on my MTB too, but the helmet wasn't involved in either case.

I don't support mandatory helmet laws.