I wear one, but am thinking of stopping. In my spills as an adult, the helmet would have been likely to prevent head damage once. - I draw this conclusion from the fact that on the one ocassion, the outershell and the underlying material were both severely damaged through my head both hitting and then dragging on the road surface.
The reason I am coming to this way of thinking, is because my elbows, knees hands and hips have taken more punishment during the spills than my head has ever done and I ask myself, "Do I want to look like a 7 year old dressed by his/her parents, for his/her first visit to a skateboard park?". The answer is no. Plus if I went for protection of the bits which have taken the brunt of my spills, my cycling would be much less defensive.
I enjoy my bike and cycling isn't a dangerous pastime. In fact my other pastime can be way more dangerous in terms of permanent effects on the body or ultimately fatal consequences.