The IT TV mini-series of course doesn’t begin to compare with the novel, which is an old song, or a cliché, for SK fans. But for people who haven’t read the book, it’s not that bad for a TV series of its time. Some of the kid scenes were probably the best, if memory serves, though the Henry Bowers in the TV series was nowhere near as menacing as the book version’s was, as Lauder from The Stand TV series was pretty flat compared to the book’s.
I agree that Shawshank was one of the best movie adaptations of King’s work; Frank Darabont seems to be the master at translating King to the screen. I also think it works better to adapt a novella or short story into a film, because you can build on it instead of being forced to either gut it or cut it up into two or three part movies (like the final Harry Potter film and the Hobbit).
You’ve hit on what is perhaps the greatest flaw in a few of King’s otherwise great novels: their endings tend to be anti-climactic, making the reader feel let down after an intense build up. The Stand comes to mind, where the Dark Man turns out to be kind of lame and curiously inept: a paper tiger of sorts. A rather disappointing villain, once you get to know him better, though King has said he believes it is often the case that evil turns out to be banal under closer scrutiny, so it seems this was intentional on his part. I first read The Stand when I was maybe 14, and had many nightmares about Flagg. I kind of relished this; I wanted Flagg to scare the hell out of me, not be deflated as he was.
Among his later novels, I didn’t think Cell was all that good (for SK), but I really liked Duma Key.