Just when you think you've seen every trick on TV... It's pretty safe to say that the core concept (not the reveal of *how*) has not been seen on TV before. However, there is a Hollywood movie that came out 2006 and it had a similar core concept (but a different "how"), so it is not like this idea is unique. Also, the Winchesters' first "solutions" to the situation recall a certain Top 250 comedy from 1993... and episode #3.11 of Supernatural itself. But that doesn't mean this is a rip-off. Roger Zelazny once commented, it isn't the story, but the way the story is told - and the story here is masterfully told.
It all starts from a "WTF?" situation, seemingly making this a Weird One - surely this cannot be a straight episode, right? Then again, it is *so* funny from the start that this could also be a Comedy One. And surely this is not a Mytharc Episode, right? Whatever it turns out to be in the end, it is first and foremost amazingly assured. So assured, in fact, that it slips in a *major* revelation about the Mytharc right under the viewers' noses. At the end of Season 5, if you remember this episode, you'll see how incredibly clever the writers were.
The title sequence is hilarious and from the opening titles we can see the guest stars. It's good to see Misha Collins back, and more Kurt Fuller is also appreciated. As newcomers to Supernatural, it's always good to see the underused Keegan Connor Tracy (Final Destination 2, the new BSG), who probably is so underused because of her distinctive uncanny doll-likeness (in a good way), and the equally underused Rob Benedict (Buffy, Felicity, Alias) is just hilarious.
This is an instant classic, as equally funny as it is significant. It's almost classic enough to warrant a 10/10, but it's structurally a bit unbalanced and with some scenes that could have been excised without affecting the whole. Also, after you've seen the season 4 and 5 finales and start thinking about how this episode and they both form a cohesive Big Picture, your head might start to hurt. So, all things considered, this is about 94% perfect. And that rounds down to 9/10. After all, the user rating for similarly funny and significant but superior #5.8 is higher.