In a word, awful!
The plot and the characters are not well developed at all. I think the writer and the director didn't have a good understanding of the book they've adapted. They have changed a masterpiece (the book) into a silly entertaining movie. It's not even frightening at all (except the first seen we see the clown). The book narrates the feare and vulnerabilities of childhood: fear of sex, menstruation, race, peer pressure, etc. in a absolutely amusing way and the clown is the soul of those fears and doesn't exist that much physically but the movie has changed it to a fantasy boring series of events that after a while you can predict what's going to happen- the clown is coming!
I think the fact that it was well sold was more horrifying than the movie itself!
In the end, I DON'T advise anybody to watch it.