Stephen King's books tend to be both very long and contain a lot of internal monologue. That's very much not film-friendly. "Faithful" adaptions tend to drag and have a lot of tell-don't-show, which makes for a "terrible" film. Unfaithful ones tend to change and cut a lot, which makes them "terrible" adaptions. For instance, "The Shining" film has very little to do with the book, but is an absolutely phenomenal movie. King hated it.

"IT" the Tim Curry version has Tim Curry in it, who was absolutely fantastic. A lot of material from the book was cut out - I'm thinking it could be 80% or more. That includes the scene where the children have a gang bang in the sewer. Out of nowhere, with no foreshadowing, and it's never mentioned again if I remember correctly. That might make it a "terrible" unfaithful adaption, but you know something? I'm alright without seeing that.