Yeah, taken as a straight horror movie, it didn't quite measure up, but as an exploration of childhood trauma, it's really clever.
I read the book as a kid and never understood why King had the characters defeat IT twice, 27 years apart. It just seemed redundant at the time, at best creating a contrast between how children and adults handle conflict.
Watching the movie helped it click for me. That's what trauma is -- it always bites twice, and the second time is a bitch.
RE: IT, Fear Response and the Spectre of Childhood Trauma - A Deeper Look