Damn, there are fantastically written reviews of this all over the place. Pardon me if this siphons off any of them. I'm basically going to reiterate everyone else's points.

Horror movies get hyped a lot, rarely meeting expectations by the time general audiences (including myself) are granted access. This is one of those miraculous exceptions. One of the finest marriages of supernatural terror and character development I may have ever seen in this genre, making it a huge success both at being frightening (you may not walk out of the theater scared for your life, but you will be haunted by nightmare imagery, visceral human trauma, and the disquieting apprehension of evil and madness lurking all around you) and being a substantial work of fiction. Director Ari Aster brilliantly wields cinematic tools to heighten all parts of this movie - the acting is unbelievable (I don't think I've ever used the term "jaw-dropping" but Toni Collette earns it in scene after scene, while Alex Wolff's performance as her beleaguered son is draining, and Gabriel Byrne provides a quenching sense of relief as the lone voice of reason), while an intense methodology informs the storytelling, dialogue, camera work, lighting, even down to the imperceptibly surreal staging and design that sometimes blurs the line between what is real and what is a dollhouse miniature model like the ones Collette's character crafts for a living.

As a horror fan, one thing I personally admired was that even as matters spiral out of control, there remained an honesty to the way the characters interact. The explanations, the emotional outbursts, the begging, the willful downplaying, the acknowledgement of skepticism, it all plays out so believably, and that never happens in genre films. I kept thinking "that's exactly how I would have behaved in that situation", even when that applied to the very different ways that each person behaved in it, which goes a long, long way toward enhancing the viewer's connection to your movie experience. It does a tremendous job of establishing sanity before making each leap into horror.

It's been nearly two weeks since I saw this, and my respect for it still grows. What was initially 4 stars/B+ but denied higher status because its traditional scariness was way over-hyped beforehand and the overall spell of the movie began to dissipate during that final segment as it marched toward an ending that has been done the same way elsewhere at least a few times already (including in another recently highly acclaimed indie horror movie that made waves at a film festival a couple years ago, and relied heavily on atmospheric dread, but I won't say what it is or else it might indirectly spoil how this one ends), has since bloomed up into 4.5-star/A or A- status because neither of those nitpicks significantly diminishes what a superlative act of filmmaking "Hereditary" is. It is destined to be cited and saluted alongside other sophisticated classics of horror like "Rosemary's Baby", "The Shining", and "The Exorcist".