Here in Australia we have had mandatory helmet laws for about a decade now, and the most noticeable thing about it is that they haven't really made people (particularly teenagers) actually wear their helmets. Many choose to simply tie the helmet to their handlebars while they ride (the logic of which escapes me, but I'm told they put their helmet on if they see a cop).
I personally always wear a helmet when I ride, but I support no legislation whatsoever to force people to do so. The reason for this is because I support Darwin's theories about "natural selection". If anyone is stupid enough to be a serious rider and not wear a helmet they might as well be left to suffer the consequences. Survivial of the smartest I say!!!