People had concerns over what would happen to Castiel, infact they down right hated the idea of him being the new bad guy, and I will admit I had my doubts, especially after the mess season 6 had been (still loved it but I'm just saying) but I think after this episode, all the doubts should be gone.
They were empathetic to his character in the end, and made it so he's still the Cas we know and love, but he's just taken over. It's like Sam and the demon blood in a way, the power went to his head but it was never really Sam.
On the topic of Sam, does anyone else notice that he's gotten hotter? Maybe its because the dog eyes are back in full force. When he was talking to Lucifer about how none of it was real and he was really in the cage being tricked by Lucifer I almost cried at the expression on Sam's face. It was heartbreaking. But I love how, despite the memories, Sam is still Sam. He was willing to try and save Cas regardless of what Cas had done to him, his sweet and caring nature is still there, and after watching him for half a season with no soul, you can totally notice.
Dean and Cas were amazing also. Cas's switching from one personality to the next was flawless and unbeliveably amazing. Dean, who is slowly breaking, was heartbreaking to watch. How he had given up on getting the real cas back. Though I am kind of sad that he didn't look for Sam and instead helped Cas. I would have loved to see that part of the bond.
This also had it's humorous moments, the porn, the conversations with death and the expression on crowley's face as he was waiting for Cas to kill him. Priceless.
All in all I cannot wait for the next episode.