Don't worry, no spoilers here. The episode was very good - not my all-time favorite, but in the top ten. That's quite an achievement for a show that's been running for nine seasons.
In my mind, Supernatural is one of the most underrated shows on television. Its viewership on the CW was close to 4 million in its first season, and has hovered between 2 and 3.5 million since. While that's not a Game of Thrones level audience, it is a remarkable level of consistency over nine years. Not to mention those numbers don't include Netflix viewers, which I suspect (based on anecdotal and unscientific evidence) add up to many more.
The unique thing about Supernatural is that it manages to combine a Kolchak: The Night Stalker-type monster-hunting series with a show about two brothers with strong personalities that put them constantly in each other's faces. While they can and will have it out on a regular basis, they always have each other's back in the end when it's time to stake the vampire (or insert your favorite monster here). And all of this is done in the context of a greater story arc which makes sense and maintains continuity.
Speaking of continuity, it's a major theme of Metafiction. And, as a writer, it was details like that I took away from the episode. One of the most remarkable things about the show is that it contains something for everyone, including writers. There are several episodes in which one can almost see that the writers are writing for other writers, and Metafiction is among the best of these.
Again, no spoilers - you'll have to watch it yourself. And if you've never seen it, well, then you get to watch the whole series first. And honestly, that makes me a little jealous.