My helmet saved my life' anecdotes prove little towards the effect of enforcing helmets on an entire population, and notwithstanding the tendency for people to exaggerate their claims. Anecdotes can be a compelling argument for individuals to choose to wear helmets, but do not constitute the scientific evidence which should be a pre-requisite to legislation. There is a rational explanation for this phenomena. Wearing a helmet increases the size and mass of the head, and helmet wearers may also be subject to risk compensation. Studies of the mechanics of head injury show that the most serious contributor to brain injury are rotational forces, which helmets can do little or nothing to prevent and may actually increase.
Helmets can have little benefit in a severe collision with a motor vehicle.
Bicycle helmets are certified only for simple falls. Don't even think about civil liberties, you don't have any. Just as compulsory motorbike helmets were used to justify compulsory seatbelts, and compulsory seatbelts in turn were used to justify compulsory bicycle helmets, there can be little doubt that at some point in the future the bicycle helmets law will be used to justify other breeches of civil liberties. The helmet law has fundamentally failed in its stated aim of reducing head injury, to say nothing of the adverse effects, but the Government has so far refused to review it. The Bicycle Helmet FAQ, from the Ontario Coalition for Better Cycling.