Before Midnight is a couple having a long discussion, or fight depending on how you see it, for two thirds of the movie. The intro is very solid, and the whole film is interesting and forces us to think about those recurring unpleasantness you can face in a relationship.
Both positions are somewhat stereotypical and the production runs a bit long, but it's real. You can see both points of view and that they both had many opportunities to quiet things down but mostly didn't because they were hurt, just like anybody else. In the end, by spending so much time on their these clashes, it makes the point, not explicitly, that it's all a bit silly.
It's great, but only for those wishing to explore the idea of love with imperfection.