The movie is about a guy who visits the family of his army buddy and is there to help them with anything. But things aren't as easy as they appear to be. That's what I'll say about the plot as this movie is best when you really don't know anything, that is how I experienced it and is probably why I liked it more.
It had a great build up, the first half or so was well directed, slowly building up the characters and plot, and then the movie releases its intentions upon you. Now I can see many people not liking what happens. It is polarizing, but I for one loved it. Was a great homage and was directed beautifully. The atmosphere feel was really great in the movie, I loved it.
The acting was good by the cast. The main dude, Dan Stevens was great. He was obedient, he was supportive, he was charming, he was ruthless and so on. A wide array of emotions, delivered greatly.
There are tons of great shots in the movie. And the best thing in it is the soundtrack. SO FRIGGIN AWESOME. 80's stuff perfectly incorporated into the movie and its themes. I had such a blast with it.
There were some character decisions and reactions that really felt stupid and there were some other stuff that I can't go into without spoiling that I wish could have been elaborated more.
All in all, this is one of those movies that just comes out of nowhere and surprises the hell out of you. And I always love it when that happens. I would really recommend it and I would also say that don't watch any trailers etc, just give it a try directly. What's the worse that can happen.