We purchased two of these to take our cats on a cross country flight.... we didn't take the flight yet but we've used the carriers a lot already. Our cats are fully grown adult females and probably weigh about 13 pounds each, and they fit just fine in these carriers. The rolling feature is great and the cats don't seem to mind the vibration of the sidewalk. The backpack feature is nice too, but I think the cats felt a little seasick from it because they started to meow a lot. They are spoiled indoor house cats but they are adapting well to their walks through the city in their new carriers. Overall, this is a well designed product, but there is an odd feature. When you lay the carrier down, the cat is laying on the thin, relatively unpadded back section and the handle bar goes right down the middle. So, the cat is pretty much laying on the handle bar and they were a little uncomfortable. We will definitely get some extra padding for the long flight.