I haven't in a while but I've still got a whole shelf of his stuff, I always found Pet Sematary scarier than It personally, but It sure had its moments. And the Dalk Half, if you know about his history as "Richard Bachman". And Needful Things, which was gleefully evil.
"It" was written at the point, if tales are accurate, that he was doing so much cocaine that cocaine should have a co-author credit and it kind of shows, it's got that hubristic expansiveness that white-powder projects usually have. Huge swathes of the (long!) book doing nothing really to advance the story just shade in the background and develop this sense of evil. I actually read a passage from It for one of my GCSE English exams back in the day (yes, for some reason we had a "reading things aloud" portion). It's a very creepy book.
I kind of think with It when you strip it down to the central idea it's so nearly there, but it needed a bit more of a clear head to push it through to it's logical conclusion. Cos as it is, he kind of goes to a pretty unforgivable place in pursuit of what basically amounts to a pun, that from all accounts the film was wise to sidestep. But Pennywise is an unforgettable villain, one of his best alongside maybe Leland Gaunt from Needful Things.