I do wear a helmet. Often times I'm the only one in the group. No one thinks it to be uncool though, or ever says a word to me about it. So, I'm glad that everyone can usually be adult about it.
I used to never wear a helmet. I've flipped an ATV twice though, luckily though I didn't suffer a head injure either time, but came very close. After that, I had a change of heart in regards to helmet wearing, and have ever since. That, plus I've had a couple of friends have severe head trauma, and acquaintances who have died from head injuries. I don't know for sure if the helmets would have helped, but I doubt that would have hurt any more.
I have both an open face and full face helmet. I've found that I prefer the full face most of the time. I feel safer with it, and the way it's made, it actually seems to channel more air in around my face so it isn't really that hot either. In the winter I can close off the head air vents, and put paper towels around the face vents, and it actually helps fight the cold.
The problem with full face helmets, is sometimes people aren't too receptive to them out on the trails. I've noticed they get nervous about not being able to see the face of the person they are passing. That's why I also purchased the open face helmet. It keeps people from being suspicious.
I've noticed too that other people will wear them when they see someone else put them on. So, that's a good thing. Kind of like leading by example.