I don't personally wear one, I do encourage my young daughter (aged 3.5) to wear one, until she is old enough to form her own opinion.
I do have a real problem, however, with any proposals to introduce the compulsory wearing of helmets (such as recent moves in Northern Ireland). My country of birth, Australia, introduced compulsory helmets when I was a child and it had a massive impact on killing off any potential increase in cycling down there. The safest countries to cycle in - Holland, Denmark, Germany, France etc – are the ones with the most cyclists, so it should be in our interests to champion freedom of choice and allow people to ride however they are most comfortable.