I used to carry on a garment bag (in coach) frequently, when I flew DC-10's on NW and United -- there is or was a special storage area for them right behind the movie screen. It was a contraption that lowered when the plane was on the ground, and then went up into the ceiling before takeoff. I haven't been on a DC-10 in nearly a decade.

However, I soon realized that the Boeing and Airbus planes did not have anything comparable in coach, so I stopped using a garment bag for coach flights. Even now in first class, I still limit myself to a 13/9/22 carryon bag, and don't have to worry too much about the wrinkles if I remember to pack wrinkle-free shirts.