How do you follow up the best episode of the season so far? By attempting to top it? Sure, that works if the best episode was "only" good or great, but when it happens to be one of best in the series *ever*, you have an almost insurmountable problem. The show-runners made a wise choice by deciding to do a Format Breaker. At first this may seem like a Weird One, such as "Monster Movie" (#4.5), but it really only breaks the series' format of "weird things happen, Sam and Dean arrive to investigate".
At three minutes, the pre-credits sequence is slightly too long, especially as the premise could have been presented in a more compact way. Well, at least there are even longer ones on TV (*nine* minutes is the longest I've ever seen). Interweaving two different central ideas into one episode - even when that's the whole point of the episode - isn't easy, and when one of them could have been just about any Monster-of-the-Week, it's not *that* impressive an achievement when the interweaving succeeds.
The good news is, there is some nice action and plenty of laughs, especially one spectacular gross-out moment with black humor. There are also some subtle self-ironic moments regarding the series format. Also, almost always when Kurt Fuller is in anything, you can safely bet he delivers one of his trademark, deliciously arrogant, crooked, untrustworthy characters. His inclusion in the recurring cast is a definite plus. And Castiel's previous revelation that not only did Dean make the breaking of the seals possible in the first place but he also has to stop Lucifer finally made the writers break Dean out of his uncharacteristic "I'm not demanding answers to why I was broken out of Hell" state, which was a really good thing. Castiel did not know the details, so now Dean has to seek the answers.
However, an episode where the cleverest thing is a couple of unexpected familiar comic relief characters, doesn't really achieve greatness. A solid 7/10 it is.