So far the cast list only consists of one name: André Holland playing a character named Henry in an episode. You think about Henry Bowers of course, but he's from Derry rather than Castle Rock.
With The Mist as well there'll be enough King on tv. And I even haven't finished Haven yet - although I plan to and am now rewatching Season 2. The first time I think I got to somewhere in Season 3. It's not that I don't like it, but there's constantly new series coming and so many in the horror/fantasy genre, it's hard to keep up with it all.
Often here on tv they start a new series, but don't finish showing the later seasons - only with the real popular ones like The Walking Dead they continue. I think a lot of the other ones don't get watched enough. So you end up buying the dvd's (which is a better way to watch anyway without commercials and things like that).
Too bad they took off SyFy here. The quality of shows on SyFy got real high with Haven, Continuum, Defiance, Dark Matter. And since they show own productions they also kept broadcasting all the seasons.