UTDestroyer, Supernatural seasons 1 - 5 run a really nice story arc and finish off quite nicely as the series was originally envisaged as a 5 season show. I think the very last scene in season 5 was tweaked as they knew that the show was going to continue beyond the original story arc. They then prolonged many of the story threads into season 6 (I won't say anymore as I don't want to give anything away!)
I hate when shows have a planned timeline and then they stretch it beyond. It's never as good because the writers suddenly have to come up with new material beyond a clean ending.