Always wear a helmet.
Had an accident last year (a pine tree jumped into my path). I collided head first and was out for 20 minutes. Suffered a cracked wrist but no head injury. That was WITH a helmet on my mellon.
If i wasnt wearing one i wouldnt be able to plan my revenge this year.
Joking aside, that shit was scary and i wouldnt think of going up a hill without my helmet afterwards.