It's all metal and very comfy when you lay down a foam or good sleeping bag. I managed to get about an hours sleep, and about 3 hours of eyes closed kind of peaceful 'rest'. Roof racks are hard and angular, but if the contents of your pack is emptied to lay on, and you wedge yourself between, say, a pack of potatoes and another person, sideways movement can be avoided. The best option is singing along with the locals or a silent retreat to the back of the bus to take in the amazing stars and avoid the chilling wind. Can't say I'd do it again.
Stools are available to sit in the isle, if their are no seats left.
My friend and I had to slept in the trunk of her car, when we went down to Ocean City for the weekend. One of the best places to sleep is actually in cemeteries. The first time was in Brattleboro, Vermont. Denny's is the place for you. When I was in college I had a Greyhound Bus Pass on spring break.