Really under-rated.
Most of the "monster" movies I've seen are not really good movies. They focus entirely on special effects, gratuitous gore, and then toss in crappy acting, unbelievable storylines and action, and end up with no substance at all. Cookie-cutter monster flicks.
This one is different. i suppose if you're just looking for straight-up blood, flesh and constant graphic overload, this film isn't for you.
I thought it was extremely well-done. The cinematography was really on, and set an emotional mood. It had the ominous feel of Door in the Floor, or, as someone else mentioned, the sadness of "Let the Right One In".
It's really all about childhood loss, maturing, and facing your fears. While some may grumble because there wasn't a top-notch, massively-detailed "monster" (which may have been due to budget, but that's probably a good thing), that really would have distracted from the real theme.
It seemed very clear to me that the real monster in the movie was the mother. The girl says her mother told her there was no such thing as monsters, and she says, "No, there are". Her mother was abusive, destructive and narcissistic, which is pretty much as close as a parent can get to being a monster. The stuffed animal that plays nursery rhymes at key terror moments drives that home, as does the plot line about the father not coming to save her. She's not a little girl any more, and she has to face her monster. In the end, she tells the monster she's not afraid of them, and then tells her mother she doesn't hate her
I thought it was really a good little movie. I usually rate most "monster" movies at 6 or lower; I gave this an 8.