I've watched several episodes, not sure how many though as they tend to blur into one another, something I see as a good thing.
I just saw one that had the guy from Fringe in it, but he spoke the same, dressed the same and looked the same. At first I thought I was watching an episode of Fringe.
I'm not sure if I like the headless horseman character, though that might be due to a story I'm writing that has many mentions of the four horsemen.
I think I said somewhere else that according to several religions the rider of the white horse was meant to be Jesus Christ, which to my mind would mean the Four Horsemen were of God.
I'd prefer it if the show had a clearer view, for instance: that the various witches were good bad or indifferent (White witches, black witches or those who use elements good and bad to get what they want).
Overall though I like the show.