This may be a mild spoiler, but I think you should know this upfront: this is not a monster movie. The monster is just a metaphor.
It's a movie about the monster inside oneself. Suppressed feelings, abuse, guilt and much more. It deals with trauma and shows this through a unique visual trick.
I understand how people come back disappointed from this movie when they expected a giant monster spectacle.
But if you're open to what the movie has to offer, you're in for a compelling story about broken people. I certainly enjoyed it a great deal.