It probably goes without saying that helmets are a good idea, but screw it, we're going to say it anyway: HELMETS ARE A GOOD IDEA!
Don't take our word for it though. Check out these pictures, compiled by Bored Panda, for a sobering list of reasons why you should always wear helmet. It doesn't matter how awesome a rider you think you are. It doesn't matter if helmets make your head sweat. It doesn't matter if it ruins your hair style. The only thing that matters is that it might very well save your life one day, or at the very least prevent your face from getting mangled beyond recognition. Scroll through the pictures below and imagine what your head might look like if you hadn't been wearing a helmet. Not a pretty sight, is it?
My face and head took the greatest impact with the tree. I took a direct hit in the face which lead the doctor to proclaim that I was lucky to be wearing a full-face helmet. It didn't crack, the visor didn't break. My cheek bones weren't crushed or bruised, my spine was fine, my neck was only slightly sprained, my teeth were intact!