I don't know what it is about boxes and cats, but they certainly do love being in them, even when they can barely fit inside. I think that sacks are also great places for cats, and I have enjoyed watching the cats struggle to get their self into a sack and then get turned back around so that they can sit inside and peek out. I like the ideas of the cat stackable boxes, and I think it IS like a habitrail for cats ! !
If a persone has more than one cat, i bet they woulld have fun going from "room to room" and following each other around in there, too.
At $15 per box, it is spendy for me; and I have amazon boxes that work and they are free.