I've run head first into trees, cars and park benches, and been hit by cars while wearing a helmet, and the helmet's have definetly helped avoid more serious injury... yet, everytime I've had a concussion, I wasn't wearing a helmet... and, after falling off a roof once, came to thinking I had been in a bike accident!
Once I woke up from a wicked drunk after doing some late night beach riding, and I was floating on my back in the shallows of Lake Superior wearing my helmet....I'm convinced my helmet saved my life that night...