Joshua Rothman, writing about the book and movie versions of IT at The New Yorker:
The novel has weight not because of its monsters but because it tells a larger story about the discovery of evil. As the kids become adults, they learn more and more about the history of Derry, Maine. They find that it once had an active chapter of the “Maine Legion of White Decency”—a version of the Ku Klux Klan—which murdered more than a hundred African-Americans by burning down a night club. They hear about the killing of a gay man down by the canal and about the gleeful vigilante execution of a group of fugitives by the town’s bloodthirsty men. Mike, who is black, is tormented by racist bullies; Beverly is horrified by the sexual advances of her father. All of these disparate evils would have existed anyway, but they are exacerbated by It—a creature that, in addition to eating children, “feeds” by fanning the flames of violence, hatred, lawlessness, racism, misogyny, and sexual predation while disguised as a clown. (Sound familiar?) It’s this evil—historical, unacknowledged, and pervasive—that is truly disgusting; the Losers Club defeats it by feeling disgusted, rather than afraid. Together, the club’s members judge It, and, animated by moral clarity, call It to account. In the final, psychic confrontation, It tries to pull their minds to a zone of mindless nihilism—that is, into an adult mindset of acceptance or repression; they refuse, dragging It into the metaphorical sunlight.
I haven't seen the new movie version of IT yet—I've got two small kids, so about the only movies I see in the theater these days involved computer-animated animals and cars—but I liked the book when I read it twenty years ago, and I enjoyed the ABC miniseries from the early 1990s. I will probably watch the new movie when it comes out on video.
Rothman is right. IT is not a scary book, at least not in the visceral way that Salem's Lot or Pet Sematary are. It is a book about being scared—what makes us scared and why, and how fear puts roots deep down in our history and culture and grows.
I have always been on the fence regarding the book's mythological/cosmic aspirations. I can appreciate what King was trying to do, and to a certain degree, it works. At a grand level, the story of It's origins and fall to earth provide an interesting narrative backstop to both the immediate goings-on of the plot as well as the small-scale historical backstory. Each kid in the Losers' Club thinks s/he is being chased by one scary thing, then they find It is after all of them, then they find out it has been living in and feeding upon the town of Derry for centuries, and finally that It is nearly as old as the universe itself.
It all does get a bit murky and muddled in the last hundred or so pages of the book, though, doesn't it? One gets the sense reading it that King had thrown everything he could think of into the pot, turned the gas up to high, and was struggling to keep it all from boiling over.