I only bother with a helmet when I'm mountain biking, as I fall off a lot.
Mountain biking is the only time I can say that a helmet might have prevented something.
I cycled under a low branch, and it hit the helmet with quite a whack. However:
-Would I have hit my head if I had not been wearing a helmet?
-If I would have hit it, would I have suffered a head injury worse than a bit of a bump?
-In any case I had a stiff neck for a couple of days
Personal anecdotes are useless in deciding whether helmets are effective.
We need to look at large datasets.
This has been done, and no significant protective effect has been detected, so either they do not protect against head injuries to any significant amount, or they do, but make it more likely to get a head injury in other situations.