I'm pro-helmets. I'd have a few less friends without them. I've seen their helmets after their crashes - they would either have been killed or ended up with brain damage without them. Some of their crashes were on roads, some were on tracks in the forest.
I never wear mine anymore (I think I lost it actually). I really should buy a helmet though, I know it's terrible not to wear one and I feel like a moron every time I think of it. I've never been in a crash but I guess the first time might be the fatal one.
I think I read an article one time that wearing a helmet ultimately doesn't make a difference...although this was mostly referring to motorcycle riders.
Um, everything I've seen (and was presented in my MSF course) certainly indicated that motorcycle helmets are flat-out life savers. There is a limit, however, to how much it can help you with a motorcycle--if you hit a concrete wall at 100MPH, for instance, helmet or not you're fucked.
I've also read reasonable testimony to the effectiveness of motorcycle gear (specially-padded/fabric'd jackets, gloves, boots & pants) in a crash.
They've been compulsory down here for a long time. I used to commute using a bicycle and do long distance touring.
I got concussion once when I was run off the road by a car side-swiping me when I was riding to University. I ended up with numerous lumps/bruises/grazes. But the only permanent thing has been tinnitus (very mild ringing in ears) long term. The old fashioned little padded leather straps "cyclist helmet" I wore saved me quite a bit of surface injury. Yet I've got this constant little reminder of my misadventure.
Dying isn't the issue for me, the bigger worry would be brain damage that impacted my ability to live independently. Then it becomes a cost to the community or your family (or worse, you are stuck without either!) I've seen enough of that with a few people I went to school with who injured themselves horse riding/cycling/skateboarding.
Also I used to ride a motorbike and through my club we used to visit injured motorbikers - sometimes the only thing unbruised/broken was their heads (that were in helmets).