Currently this show has a 5.6 rating and is getting abysmal reviews from people. Most complains are about editing, flashbacks, non-linear storytelling, casting etc etc.
For a TV series, it is extremely well shot and photographed. Non-linear story telling should not be a problem as we got a well known story (even for people who had not read the book will get it easily). For those elements it should easily get 8/10.
The show lacks proper casting. James Marsden - while he's fine as an actor - is somehow gazing into the great infinite in some scenes. Whoopi is overacting, Owen Teague is sometimes excellent and sometimes bad, can't tell why. Mr. Skarsgård is missing something as Flagg, again can't tell what exactly. Greg Kinnear is excellent, lights year away from all those. So 6(-)/10 for casting/acting.
Check out other recent series with a 5.6 rating. This show is way better than that.