Personally I use newspaper, but I've tried just about every substrate there is at one time or another. The problems I had with the carpet is that you do have to scrub them and soak them in a disinfectant solution fairly often or they can get stinky pretty quick. You should have at least two carpets per cage so one can thoroughly dry and air out before you have to clean the other one. Also, while I never had this problem with ball pythons, I kept some leopard geckos on the reptile carpet. Often when I fed them crickets, they'd pounce on one and miss and end up getting their teeth caught in the carpet fibers. Then I'd have to go in and pry their mouths off of the carpet. (this was a carpet that was made of compressed fibers) pretty strange.
Mark