I grew up in New Zealand. The interesting part for me is the social uptake of this law - it was universally accepted and we were always told to wear helmets. I don't think I ever saw anybody there riding a bike without a helmet.
People don't ride bicycles in New Zealand because they have to wear helmets, they don't ride them (or stop there kids from riding them), because the roads and drivers are absolutely terrible. The drivers literally just push on through like you aren't there.
If you say to anyone you ride to work, the first question you usually get is "is it dangerous? do you have a good route?". Thats how people feel about cycling, if you do it, then you might get seriously hurt.
Same thing in London by the way, I used to ride to work there. I got the same style of response "Jeez, you're brave", "Roads are to dangerous for me to try that". I felt way safer in London than I ever did in New Zealand though. Something about constant traffic jams that lets you slip right through :)