I join a night group ride every Monday for about 30-35 miles and I love it. It's a great way to enjoy the road with your focus on the road. As a training ride, you really learn to keep your attention close to the bike, within the distance of your headlight. It's not terribly easy as there are no street lights out there and your headlight is all you have.
For a while, I mounted my head light on my helmet but it was too difficult to see the texture of the road. Now I under-hang my light from the bar and lots of shadows appear quite nicely.
I'm especially looking forward to riding in the snow at night.
Night rides for sure! As a father of three small children if I want to get any decent mileage in, it's night rides or nothing. I've found that if I can only have one light the helmet light is the way to go but nothing beats the helmet/ bar light combo.