As a cyclist, I am convinced that my helmet has saved me from at least one significant head injury: the right side of my helmet still bears the scars of bouncing off Yarnallton Road about 5 years ago. Better scars on the helmet than my head.
But a story in the Washington Post reviews studies that have yielded mixed conclusions as to whether mandatory bicycle helmet rules really promote bicyclist safety: Do Bike Helmet Laws Really Save People? One disturbing fact discussed in the story is evidence that motorists become more careless driving near bicyclists when the cyclist is wearing a helmet.