Longlegs is an unfortunate example of a marketing campaign working a little too well. The expectations and hype around this movie set it up to fail in the eyes of those who aren't into more nuanced, slow burn, thriller types of horror. I think the vague strategies used to promote this movie led many into expecting something it was never intended to be.
If you're expecting this film to be akin to Insidious, The Conjuring, etc. you aren't going to get that style of horror here. Although it does contain supernatural elements, the movie itself is primarily a mystery/thriller that establishes it's creepiness by crafted camera work and an ever present feeling of dread. There's a few jump-scares in it from time to time, but it's not a "oh no, don't go in that basement I just know something will pop out at you!" experience.
What Longlegs does best is establish a feeling of dread that sticks with the audience until (and even past) the very end. The dark hallways and abandoned farmhouses aren't scary because we know there's a ghost in them- they are scary because there's a small chance something COULD be there. It's done best when moments like that are sprinkled into the film from beginning to end, and as a viewer it makes you that much more engaged with the screen at all times. I found myself constantly looking in the background of scenes, and Oz Perkins uses camera angles and wide shots to perfection here. You'll find yourself gradually fearing things that never end up being there, creating that uncanny valley type of dread that all of us have had at one point or another.
It's like when you wake up in the middle of the night to get a drink of water or use the bathroom at your house. In your mind you know you're completely safe, you're at your own home and you never heard/saw anything that would suggest otherwise (unexplained noise, open windows that were closed, etc.) And yet, for some reason you start to feel off, as if there is some sort of danger you'd been unaware of. That urge that makes you open the shower curtain for good measure, even though you had no logical reasoning for thinking there really was something behind it.
This movie isn't perfect, I thought the third act could have been stronger. And if you are the kind of viewer that wants no plot-holes and sound logic in their movies, then you probably won't enjoy this one as much. But if you're like me, someone that can embrace the suspension of disbelief and immerse themselves fully into the atmosphere of a film- it is well worth a watch. I also think this movie should be seen in theater if possible, because it works best on a large screen where the cinematography and sound design flourish the most.
Above all else, don't listen to these big critics calling it the best horror movie of the decade. Don't let them set that expectation for you because you will be let down in the end. Go into with an open mind, be willing to embrace the atmosphere and plot, and I think you'll find this thriller/horror movie to be quite enjoyable. At the very least, it can keep you on your toes and have you at the edge of your seat until the final act. Plus- who doesn't want to see Nic Cage finally get to act like a deranged serial killer in a film where his character actually is a deranged serial killer?