If you are doing downhill mountain biking, then I would wear a full motorbike style helmet.
But for riding on the road there simply is no evidence that they are effective.
There are lots of people who say 'but my helmet broke'. They aren't supposed to break! They are supposed to crush, that is how they work.
Regarding the NEJM study
www.cyclehelmets.org/1068.html
"Those who have taken the trouble to analyse the paper in detail, however, have found it to be seriously flawed and its conclusions untenable. Moreover, by making different - but no less valid - assumptions, the conclusions change radically."
"As well as having a helmet wearing rate 7 times that of the cyclists riding round Seattle, the ?community control? group came from higher income households and had parents with higher educational levels. The observational survey of child cyclists riding in Seattle found that helmet wearers were predominantly white, middle class, riding with their parents in parks, whereas the non-wearers were more often black or other races riding alone on busy city streets. The risk profile of these two groups would be quite different."
If you are going to wear a helmet, why not wear a motorcycle helmet? A silly plastic hat, as worn by many cyclists, is clearly not as good. If you are so worried about your safety, you should obviously wear a giant motorcycle helmet. Anything less is irresponsible. [insert further blather here]