We used one last summer on our trip to WDW. Never again! It kept opening, the flaps kept hitting and we had to remove it at night during our stopover.
A funny story though: We were traveling through Georgia trying to make up for lost time when a car pulls up next to us with people waving. It was like a scene from Tommy Boy or Trains, Planes and Automobiles! We started waving back!They were waving at us because the canvass roof pack was wide open and things were flying out. We purposely didn't have valuables in there because of all the problems. We wound up losing 2 packages of diapers and some food! But, we joke about it now!