This wasdefinitely one of my favorite episodes, it simply had everything I like about Supernatural (exept Castiel being his Angel self).
You have Mark Pellegrino as Lucifer, doing a wonderfull job at being awesome and creepy at the same time. Hegave me the same chills as IT gave me when I first saw it.Here's me hoping the writers find a way to keep him in the show for a while!
Then there is Sam, poor Sammy! Jared portrays the slow mental breakdown very well. And because we have known the characters for so long, most of the time a facial expression is enough to let us know precisely how Sam feels. Btw, I'm a psychologist and I found the portrayal very convincing and believable.
Dean, allways the brave one, tries his best to stay strong, but everyone has is breaking point. I'm afraid Dean is going to crack a few episodes down the road. Jensen was great. My favorite moment was where he convinced Sam that he was real (on of the best brother-moments) and when he desperately called Bobby.
What I found clever was how the writers handled the fake Dean situation. When I rewatched the episode, you can see Sam looking confused to the keys in his hand, and when Dean arrives, there is Bobby's truck with the lights on.
I can't believe this episode is rated so low! I found it so much better than the premiere, which in itself was already fantastic. Maybe it's just the people who don't like the Castiel developments who downgrade the episode :S Come on people, no way he's dead (who else can keep Crowley in line?). And even if he is: I liked Castiel but I find it more sad for the actor Misha. I became to consider him as a core cast member. Whatever the case may be: if you like a plot twist or not, in this case it has nothing to do with the quality of the episode!!