I truly enjoyed this movie. Despite what a few obtuse and inattentive reviewers wrote, it has a pretty clear and well rounded plot that the spectator gets to fully grasp in the third act.
This is a psychological thriller in the style of David Cronenberg, so it won't please those who expect a gore fest and fast paced action.
What it does really well, which most horror movies don't do, is it makes the viewer connect emotionally with the protagonist early on, capturing her grief and her sense of isolation, so you do care about what happens to her.
The concept of a menacing specie that some twisted human is breeding in secret to preserve its existence at the expense of human lives is relatively original and well delivered.
The visuals and the soundtrack are impressive, and I saw solid performances from the whole cast, especially Hunter Schafer as Gretchen.
The one thing I found odd and confusing is the time period: the technology seems to suggest we're in the early 80s...but then the protagonist (and ONLY the protagonist) has a smartphone? I was waiting the whole time for a big reveal that would explain that, but it never came. Oh well.
*SPOILER ALERT* The key point in this story is that Gretchen knows her half sister (Alma) is the surviving twin from vanishing twin syndrome, she mentions this early on while talking to the receptionist. So when she finds out what Alma supposedly is, and how she was conceived, she believes that there was a human twin and a cuckoo twin in the womb. And that the human twin is the one who survived. That is why she refuses to believe the policeman who tells her "That is not your sister". However, in the standoff scene at the end, Alma reveals her true nature to save Gretchen. And they accept each other.
I especially appreciate the ear wiggle thing. Since they both have that, it is subtly implied that Gretchen may not be human either.