I think they are a waste of money really.
The laundry bag that you put in the washing machine is meant to be so you don't get bits of fluff on your other clothes but I've never noticed a lint problem so I don't see why you need a bag to stop that. And if it's meant to be a bag that you can put in the wash it's got to be a mesh fabric and why would you want that in contact with your door with pee against the door
But if its a bag made of PUL for the door, that is a little less nutty but then why not just dump the nappies in a bucket or if like me straight into the washing machine?
www.babyharmony.co.nz have the mesh laundry bag from memory and honeychild did have the fuzzi's door hanging PUL one. I haven't checked if they still do