I agree. Castiel is the best thing to come out of Supernatural. His character also gets to change and do new things verse Sam and Dean being stuck in their patterns. I would watch the show over the summer after a season comes out, though verse stopping forever.
While I love Sam and Dean (and many other side characters) each for different reasons, to me, there is just something so special about Castiel when he is written well. To me, he is one of the most interesting and complex characters I have seen on TV.
He is this eons old spanner in the works. He has rebelled over and over again against programming that is supposed to be unbreakable, and always for love of humanity (and specifically Dean Winchester). We have seen him in every capacity from hammer to friend, from god to homeless human. And with each of these, he grows. He learned so much from Dean about humanity and free will and, bless him, he tried to spread that message to his angel family, who he forever trying to save. When we first met Cas he was like a hammer, just a feelingless warrior of god - or so we thought. We have seen him grow to love bees, PB&J, coffee, and binge junk TV. Gosh! I just love him!