I think lightweight cycle helmets are capable of protecting you from cuts, grazes and nasty bruises, but not much more. I think the worrying thing about them is the research that suggests vehicle drivers give less room to cyclists wearing helmets, perhaps because they sub-consciously perceive them as being less vulnerable. So helmeted cyclist might be more likely than an un-helmeted cyclists to be involved in a collision.
This is a bit like providing soldiers hi-vis bullet proof vests, offering them more protection from bullets, but making them more likely to be shot in the first place.
Other research shows that while helmet use has increased significantly, serious head injuries per capita have not, which suggest that if an impact is hard enough to cause a serious head injury, a cycle helmet has little or no effect.
In addition, we all know exactly how big our head is, we don't often get it wrong and instinctively duck under things, or move our head out of the way. Wearing a helmet makes your head bigger and messes up this instinct. Anecdotally: I was talking to a friend, who is a builder who said that when working in tight spaces he's always bumping his head while wearing a hard hat, whereas he hardly ever does when not wearing one.
Personally I think there's a need for a lot more research and I really don't want to see them become compulsory, because I think their efficacy is so debatable at present. Whereas, in places where they are compulsory, research suggests that the number of people using a bicycle drops by as much as 40% and that those that do continue to cycle are more at risk, because cyclists tend to be safer where other road users are more used to encountering them.
I'm also concerned by examples of reduced compensation payments to non helmet wearers and reduced penalties to those that injure them while there is such poor and inconsistent research into how effective they are. In addition, before reducing compensation, or penalties, you would have to decide on each individual case whether a helmet would have altered the outcome at all, after all if someone gets a broken leg, but no head injury, whether or not they are wearing a helmet is surely not significant.