Finally got round to seeing this last night and I loved it.
I don't know if I would class it as a horror film. It was unsettling at times, yes, but apart from one jump scare there was nothing that I really found scary or terrifying.
What made the film so good for me was the way the story was structured, the atmosphere, and the performances by all the leads. The slow reveal of what was going on pushed the mystery as far as it could and for the vast majority of the film, I was glued to the screen. Cregger is excellent at invoking a sense of unease/dread through the sound effects, pacing and camera work, and it was used to great effect here. Garner, Brolin, Wong & Ehrenreich were all great but special shout out to Amy Madigan who absolutely nailed her character. It was just a fun movie to watch and how they tied together all the different characters' solo stories really enhanced the film.