they've signed on for a sixth season. also, note that they haven't really done much of anything with all of the open plotpoints via the apocalypse. the antichrist? yeah, he's off doing whatever the hell it is that he do. slugfest between brothers (sam/dean, lucifer/michael). god doesn't care what goes on--there has to be more to that plotline than they've shown so far.
I'm a bit disappointed in this season--they could've done SO much more with the ****ing apocalypse. but, it hasn't been HORRIBLE by any means... just a bit less than I was expecting.