I'm so happy I literally don't even know what to do. I cannot begin to describe to you how much I adored this episode. I was expecting it to be funny, and it was... but it was so much more than that. Let's jump right in.
Cons:
A teeny tiny complaint is that they didn't mention Cas at all. The only reason I'm even saying anything is because I absolutely love it when they toss his name in there at least once an episode, even if it's just to say "Oh, Cas is off doing research at such-and-such location." That's all I need to be totally content, so I always notice when they can't be bothered with even that much.
The other complaint is a bit more... complicated. Dean was kind of a jerk in this episode. I think what they were trying to show was that Dean's reluctance to accept the imaginary friends was because he was uncomfortable facing the truth that he hadn't always been there for Sam. But he just came across as kind of a dick, and I thought they could have toned it down a bit, or shown more of the complexities of Dean's feelings before the ending.