I really enjoyed the first 5 seasons of this show, and for all intents and purposes it really ended there. Everything after that is just sort of meh. Wife still watches the show, and I'll join her on occasion, but it certainly isn't the same.
One thing I think this show really suffers from is escalation. The writers felt they had to introduce progressively more powerful villains for Sam and Dean to fight, and thus had to create increasingly contrived ways for them to win.
The writers should have kept it on a smaller scale, minus the Michael and Lucifer story arc, and kept Sam and Dean's opponents more their level except in cases of divine help which should have been used sparingly.
They should have kept the Colt the only thing able to kill higher level beings, made a distinction between low level demons as being just tortured human souls and high level demons being fallen angels (which I believe they hint at but never explicitly state). Ruby's knife and Archangel swords should never have been put in the show and the death of even a single Angel/Fallen Angel/God should have been major story arcs that entire seasons were centered around, not inconsequential things that occur during random episodes. With the exception being the episode where Lucifer tears shit up in that meeting of the gods.
Also, they've made characters dying pretty meaningless. Sam, Dean, Bobby and Castiel have died an absurd amount of times that it means fuck all at this point.
As far as next season being the last season, the new head of the show says he has constructed a story arc to carry them through season 10. Though I guess the actors could choose not to renew their contracts.
