I loved Misery, The Green Mile, Apt Pupil, The Shawshank Redemption, Stand by Me, Carrie, Dolores Claiborne, IT and The Stand. All these films were faithful to the literary adaptation and Misery even won two Oscars. It's tricky to translate the unique sense of dread that King evokes in his novels to the screen but when they got it right, they really got it right.
On the other hand, every author is entitled to a few terrible books and in the case of Stephen King, i HATED The Tommyknockers, both the book and movie. Sluggish pace, confusing storyline and very labored in places, it was definitely his worst book in my opinion and the movie added insult to injury with some woeful acting. The Shining movie left me cold as Kubrick cut at least 50% of the story away and tacked on a very weak ending. Books like Bag of Bones, Pet Cemetery and Christine are also very poor and the films of Pet Cemetery and Christine were lacking.
I was most surprised by how good the tv adaptation of The Stand was. Obviously a 3 hour movie could never do justice to King's 1000+ page tour de force, so the story was split for a mini-series which totalled over 9 hours in length. Some parts of the book had to be trimmed away but by and large, the sense of terror and isolation captured by the book translates perfectly.
Many more stories could still be adapted into movies as King has a huge back collection of literature that would make great movies.