Might as well be an interactive movie with how much you should be engaged while watching this (over and over to get full value). It's a movie made for interpretation. It isn't supposed to have a clear meaning, rather, it's art for you to have an experience with. It isn't about what you watch, it's about how you think about and experience what you are watching.
In the movie, there is a character who mocks abstract art, saying anyone can do it. What is more interesting to him is paintings/art that are photorealistic - they show you exactly what they mean, and he likes that. I have to wonder if this character has a moment of meta-commentary on how people perceive this movie. Was that character was calling out people who want the movie to be more prescriptive, who want the movie to be "less abstract, more photorealism"? He says, "How can I know what to experience without someone in the art, experiencing it for me, ie. being sad to know the painting is sad?" and she responds, "Well, what if you, outside of the painting, are the one experiencing it, without needing to see a person experience it?". That is this movie in a nut shell. You have to experience it, you can't rely on the characters of the movie to hand you your experience.
It's brilliant because it's an entirely different movie the more you watch it. The movie holds the viewers hand enough to give them something to follow the first time, but lets go enough to leave room for understanding and expansion on the theories in the movie for the 2nd+ watch. You have to admire the ability to make elements of the movie juuust the right amount of obscure to make them come together over each viewing, rather than be right in front of your face the whole time.
But the movie is not for everyone. If you like Memento and Shutter Island, movies with replay value that you can interpret differently each time, then you should like this one.