Can't resist taking this bait, but I feel like the consensus is pretty much that as an individual, choosing to wear a helmet will make you safer, and as a society, mandating bike helmets (and other measures that will cause people to use transportation methods that are more dangerous to others) will make everyone less safe.
Of course, it's hard to prove. But I think you'll generally find that, if you compare the number of injuries/deaths while cycling in countries with mandatory helmets per km will be higher than it is in the Netherlands, where they are not mandatory.
