I was told by a friend; watch all the way up through season 5, then sit for a couple weeks and don't watch it for a while. Really think about if you want to keep going. Because the writers originally wrote the first 5 seasons to be a self contained story and it worked really well. Then the show was very successful, and they asked them to keep going. What ended up happening was they'd have single or double season long plots that just kinda rolled from one to another and had a bunch of escalation and flanderization going on. Like, in one season the big issue is The Four Horsemen of the Apocalypse. They deal with three of them, and then Death is the big badass. He's the one they can't actually beat. They spend an entire season leading up to him and trying to deal with him.

Then, the very next season, that same super badass, Death himself, is in the way of something they want. Summoning, binding, and killing permanently this mega badass end season boss is all the effort of half of a fucking episode in the next season. The Flanderization is most obvious in Dean, where he goes from a dude who is characterized by typically masculine traits, and emphasizes that masculinity at times, to "Me Dean, me want burger! Me want pie!!"