Very nice episode. The Men of Letters idea is a fantastic addition to the mythology and I can't wait to see what they do with it. While I've said that I'd be happy to see a return to the original concept of episodic stories involving American folklore, I'm just as happy to see the situation evolve into something new-- the guys deserve some sort of forward motion, some sort of reward for what they've done and some assets to work with. It also works with the idea that the Winchesters are destined to be some sort of game changer for the dynamic between Heaven and Hell.
The only thing that felt awkward was that Castiel was never even mentioned. I know he can't become a Deus Ex Machina, but in a situation as extreme as this, his name should have come up at least.