This film is one of the most frightening films I have ever seen, but it's like a car crash (and I mean no disrespect in that manner), you cannot take your eyes off it. It's an amazing, dark, wonderful, threatening, hopeful, depressing movie.
It's so well done and when it finishes you cannot believe what you just watched. Required viewing, highly recommended.
This also introduced me to the late Kit Gleave, who plays Duffer, and found out he wrote a trilogy of fantasy novels which are available here on Amazon.co.uk. Buy those after watching this film.