I did see Hereditary this past weekend and absolutely loved it. I highly recommend it to anyone who likes intelligent horror. It's definitely one of the best "adult horror" of the past few years (I would include It Follows, Get Out, Don't Breathe, and The Witch in that same category).
The acting is excellent, particularly Toni Colette who goes all out in this: she deserves an Oscar for her incredible performance. The story plays out like a particularly extreme family dysfunction drama about grief, with the added layer of supernatural forces exacerbating what is an already messed-up dynamic between the various characters. I don't want to go into too much details plot wise, as I feel that this is the kind of movie that works best when approached without any idea of what it is about.
It is as much a mystery as it is a horror piece. I found myself trying to understand what the hell was going on for most of the film, and fortunately I felt that the explanation at the end is fairly satisfying, which isn't always the case with these types of movies (hello M. Night Shyamalan!). This is a very impressive directorial debut, where all the pieces of the puzzle are methodically put together. I'm actually curious to watch it again at some point, as I feel that there are a lot of subtle hints throughout the film that can be picked up on second viewing.