I saw the movie waaaay before I ever read the book. I was going through my romance phase and so Stephen King books were not on my radar at the time. I remember the movie scaring me so much I was afraid to go upstairs to the women's bathroom, I was watching it in a dorm, without someone else coming with me. Unfortunately they were a bunch of guys and I begged my best friend to stand with the bathroom door open so nothing could sneak in to kill me, he was not happy about it. LOL. To this day I do not park near sewer drains and I make a wide berth around them when walking down the street. I was already afraid of clowns thanks to the scene in Poltergeist with that clown under the bed, but this solidified it for me.
Needless to say I did eventually read IT and that started me down the Stephen King/ Dean Kootz reading pathway to which I believe my fear of the dark, closed closets and rampant insomnia are attributed.
I loved the book, and even though it has now been 20 years since I first saw the movie, I cannot watch it after dark.