I wasn't expecting much from this but was pleasantly surprised, this movie is quite a gem. It's very original and interesting, the pacing is good and the practical effects and CGI are quite competent. Unlike a lot of other indie films in the same budget range it's never boring, and it always shocks and intrigues you into wanting to see what happens next.
The characters are very well written and believable. There's a lot of bizarre and frightening things happening, but they seem to accept it all as normal because they don't know any different. They have no understanding of the outside world because they've been isolated and kept ignorant. You actually care what happens to them as their characters develop and they learn horrible truths about their existence.
If you like indie sci-fi or horror films you'll probably like this one. It's relentlessly weird and bleak, and it will keep you interested till the end. I will definitely be watching the next film these guys make.