The book, I believe, is way beyond a simple horror novel. It explains a lot, I think, about a very specific part of American society and culture. I loved all the allusions to US pop culture, bands hardly ever mentionned nowadays but that were glories in the 50s, the comic-books stuff and the Muppets quoted, including the History Textbook aspect of IT. It is quite complete, I believe.
From a technical point of view, I have this theory that one of the amazing appeals of the book lies in its apparent composition as a series of short-stories that ultimately mingle together. Every story, one after the other, fits with the previous ones, and somehow create a bigger and more powerful whole. In this respect, I think the structure is simply brilliant - and some details are I think extremely complex in the sense that they make you sit and think (the dirty magazine the Losers find in the house on Neibolt street for instance).
On the purely horror part, and all gruesomeness aside, I think the murder of Patrick's little brother is one of the creepiest page I've ever read. Putting the reader inside a psychopath's brain is a tour de force in itself. Stan's confrontation with the dead boy also scares the bejesus out of me every time.
But I also think that IT is a magnificent novel in many other more psychological aspects. Emotionally speaking, and I would not like to sound too much of a sissy here, I was on the verge of shedding a few tears during the description of Bill's emotions right after Geroge's death, when his parents seem to lose all interest in him. Same thing during the beginning of the Ritual of Chud, when Stan breaks into tears because of the dirt rather than the fright. I also found excellent the depiction of "love" around twelve years old, especially from Ben's point of view. The descriptions in the Grownups part reach, in my opinion, perfection. How is it possible to describe two different moments of the same town at the same time, from the point of view of someone who comes back twenty-seven years later? Looks like a puzzle to me, yet SK manages it brilliantly. I relate strongly to Mike: his role as a watchful eye left behind, the fact that he is a kind of bringer of bad tidings, his calm modesty, all these aspects of his character make me think of him as a true hero in the background.
Moreover, IT makes me think. Mile-high. On the meaning of age, the sense of memory, the power of imagination, the "special way we have of looking the other way", and above all the concept of IT having an unthinkable "real" shape that everyone eludes behind his subjective point of view. Husserl would have loved it (if I may presume so).