There's just something distinctive about a Ben Edlund script: that absurd, twisted sense of humor that has been evident since his creation of "The Tick" through his days on "Angel". Edlund has contributed some great material for "Supernatural", and this is no exception. As "special episodes" go, this is one of the better attempts in recent memory.
As is typical for this kind of episode, certain character traits are "enhanced", so to speak. So while Dean is usually chasing skirts and Sam is usually the quiet, brooding one, the writers turn that up a notch. Dean's pursuit of the "bar wench" is great fun, if only because it's so damned predictable. (And only Dean Winchester could make those lines work.)
One nice touch, however, was the resolution to the episode. It's status quo for the Brothers Winchester to rescue the damsel in distress, and it would have fit the classic monster movie mold for that trend to continue. So it was a nice touch for Jamie to be a lot tougher than she seemed and take an active role in her own deliverance.
It wouldn't be "Supernatural", of course, if the humor and style wasn't bolstered by some strong character work and a deeper message. This episode provides another look into Dean's post-perdition psychology, and an aspect that hasn't been discussed previously. Or rather, not in this context. Dean has regained the sense of purpose that he had before his life became defined by his demonic death sentence. One could say that even when he was obsessed with hunting as a distraction in the third season, on the inside, it wasn't about the mission, it was about retaining his sanity.
In this rare moment of self-reflection (something he simply won't do under normal circumstances when Sam's around), Dean recognizes that his dealings with Castiel and God's decision to have him pulled out of Hell represent an important change in his life. His well-established atheism is breaking down. That this admission is given to a young woman he's trying to sleep with is typically ironic, but it does make one wonder: will Dean's growing faith juxtapose with Sam's descent? It certainly seems possible.
This episode also touches on the same concept explored in "Metamorphosis": the role of choice and free will. The shapeshifter in this episode was treated like a monster, so he came to the conclusion that the oppression gave him the license to kill without remorse. It's the kind of example that Gordon and Travis would love to see, since it would have made their case for them.
The bottom line is that the shapeshifter made the choice to kill and made the choice to kidnap Jamie. He felt that it was necessary and inevitable, but he could have made the choice to live a normal and nonviolent life. He was a monster because he chose to be a monster. His choice was tragic, but only because of how easily it could have been avoided. (Not to mention how pathetic the shapeshifter's life had ultimately become.)
The style of the episode also deserves mention. The homage to the classic horror movies could have been unbelievably corny, but that line was never crossed, thanks to the nature of the comedic choices. Neither the visual style nor the dark comedy is something I would want to see each and every week, but in this case, it was a brilliant move and produced a great hour of entertainment.