I've used it for rabbits, other four-legged critters and like montgomery said, it does tend to get hard if it gets the least bit damp. I did a search for "recycled cardboard substrate" on the forums and got two links back. I looked at both of them and this seems to be the most pertinent from what little of it I read.

Hope this helps: http://ball-pythons.net/forums/showt...oard+substrate