It's easy to see how flawed this movie is. Muschietti's film doesn't try to play with the audience's head. What you thought was happening is what's happening. There's no twists, no mystery, no wondering about what's going on. In this way, the technique of the jump scare becomes a metaphor for the entire film - you know it's coming, you know you're going to jump, and you jump, because that's how humans work and Muschietti exploits it. Only at the end does the movie throw a curve ball, but even then it's one you ought to know was coming provided you've watched enough of horror to know the genre.
Having said all that, there's a sort of primal brutality to this movie that gets past the linearity and the reveal five minutes in. It *is* scary in a 'holy shit' sort of way, and it is unique in that it doesn't have the normal ramp up before the ghost starts attacking. The designers of the ghost did a great job, and that saves it in the end - because in the end, this movie is about the ghost. The other characters are underdeveloped, and I didn't feel the chemistry between Jessica Chastain's character and the children till the last ten minutes. This is unfortunate, because the entire movie is built on top of this, and one of the major storyline developments - the children's divided loyalty - becomes emotionally unsatisfying because of it. Still, I have to give props to Megan Charpentier for pulling off a decent performance as the conflicted older sister. She made the whole movie work for me at a caring about the characters level.