I loved Castiel. Very well cast. They made him seem very awesome (in the sense of inspiring awe), scary, and unknown. He's very much a wildcard. I have no sense at all what's coming next, and I love it.
My vote is that Supernatural's God is actually quite different from the Christian God that most people think of when they think of angels. The portrayal of Castiel as being scarier than demons leans towards that. The fact that we have at least one demon who seems to be on the Winchesters' side and that we have a frightening angel seems to suggest that things are grayer than they seem. I suspect we're heading down the road of the moral dichotomy portrayed in Babylon 5, where one side may seem to represent good and the other evil, but in the end both sides end up being manipulative overlords that think they want what's best for us.
I'm guessing there's really no black and white here, just slightly darker or lighter shades of gray.