The Medical Journal of Australia has released a report on the use of helmets by motorcyclists & bicyclists. Spoiler alert: they think helmets are a good idea.
The risks of severe head injury were more than five times higher in cyclists not wearing a helmet compared to helmeted ones, and more than three times higher in motorcyclists not wearing a helmet at the time of injury.
https://theconversation.com/bike-helmets-an-emergency-doctors-perspective-13935
Not that this settles the question forever: other studies have indicated a counterintuitive conclusion, that helmets (at least on bicyclists) make riders & the drivers hitting them more careless, & furthermore by discouraging riding helmets increase other health risks due to lack of exercise.
http://en.wikipedia.org/wiki/Bicycle_helmet#Undesirable_effects_of_helmet_use
But still. Be careful out there!