Just saw this episode again on a TNT repeat. I forgot about it, frankly. I remember disliking it because as others have mentioned, it wasn't a Winchester story. In fact, it felt like one of those Supernatural episodes that you find out was supposed to be a back door pilot. I don't think that is the case here, but I think I might have liked it if this had become its own show. The detective is intriguing and well cast. Almost perfectly Gothic noir. Reminded me a LOT of Mike Carey's LUCIFER comic (that happens to me a lot watching Supernatural).
But the problem is that it feels so OFF as a Supernatural episode. We're told these characters have history with the boys, and that right there for us die hards is frustrating. Because we're sitting there trying to remember and it turns out it is a one and done situation, not a call back.
We're also asked to care about characters to a level I think is unrealistic since we don't know them. We're not invested. It's as if we're supposed to be just because we are TOLD the Winchesters are invested (at least Sam is).
Without getting all rambling, I'll just say it felt like a half baked premise that could have been so much more. Last few seasons we keep hearing how CW is trying to get a Supernatural spin off going and I'm thinking, hell, this would have been a good candidate. Just give us a couple more episodes with these people and then let them go on their merry way.
That ship sailed, I suppose.