One of the funny things about the series is a fair bit of the 'mythology' that the fans cared too much about (19, Randall Flagg, The Crimson King) don't actually appear until quite late in the series. I can't even remember what the '19' thing was even about.
The series is at its best early on where the world it's set in is strange and mysterious, with him teasing you with what the real nature of it is. But King has a Spielbergian need to explain everything to death so that mysterious atmosphere doesn't last too long.
But I maintain (not having reread them admittedly), that the second two books are highly enjoyable fantasy adventure books, and the series overall is weird enough to be worth reading.