Something that I noticed wasn't mentioned is, if you decide to put it inside your tank, make sure that the hole for the cord is WELL sealed around the cord (aquarium sealant works great...buy it from the local hardware store, it's much cheaper than at the pet shop) or your snake WILL find a way to escape (if it's small enough).
FYI: I have a bunch of glass tanks...with the UTH's stuck on directly. I have always (CAREFULLY) peeled them off and moved them if necessary...and never had one not work or go out on me.