it's a sci-fi movie without any no brainer special effects or weird looking aliens. It only evolves around the story and the characters and it manages to keep u engaged till the end. And that's why it's brilliant. It poses some very interesting questions and
it actually makes you think...it doesn't give you everything on screen.
I watched it knowing only that it was a movie I had to watch without knowing anything about the story. And that's how it should be, cause it's strong point is the story and you shouldn't give that away.
I've seen it described as sci-fi for thinkers and I agree. You don't have to be a good listener, you have to be willing to simply think.