Helmet use is mandatory for adults here in Seattle. I don't know how common such laws are in other areas.
I haven't had a bicycle since the law was introduced - not because of the law, specifically, but I feel grouchy about the idea of being forced to wear a helmet while doing nothing more dramatic than tootling around my neighborhood, and the law has therefore helped reduce my enthusiasm for bicycling generally.
The article in question is about mandatory use in bicycle racing.