Spoilers!!!
I honestly absolutely loved this movie. I feel like most of the reviews are people that just didn't understand it at all. If you're looking for jump scares or something fast paced then this definitely isn't for you. It really is a slow burn that leaves a sense of dread after the movie. There is an explanation for practically ever thing in the movie. Why Maika Monroe is "psychic", why she acts as oddly as she does, why Longlegs is doing what he is doing. If you believe in the Devil then this is much creepier as well. Also the set pieces and cinematography were great.
The scariest thing about the and the biggest spoiler is that it's literally just Satan toying with people because he can. The Devil is just playing with all of these people and destroying lives because he finds enjoyment in it.
My one complaint is the silver balls. I'm not sure why they were necessary and the dolls themselves weren't enough. Maybe they represent something that went over my head.