You have seen movies like this one. They are based on conversation, with a low budget and a few sets. Their strongest part is the idea and the feelings of the characters. Basically it is a filmed play.
However, unlike many of the movies in this category, this film is a great one. It's like the story of my breakup, but with a happy ending. Do they (a girl and a boy, always they are) remain together happily every after? It doesn't really matter. The ending is just as smart as the rest of the film.
All the characters play well. Chris Evans is both scary and pitiful in the role of this insanely in love guy, who can't control his jealousy, even if he is a nice non violent guy. Jessica Biel looks hot as always, but even if her character gives the name of the movie, she is not the main character and she really has a small part. Jason Statham has hair in this film! If anything else, this is something to see :) And he plays beautifully. Maybe he is tired of silly action movies. Joy Bryant plays a small part, just looking gorgeous and being a good friend.
Bottom line: a 25-35 demographic movie. I can't imagine kids of under 25 really getting it, although they most desperately should. It's a romantic film of sorts, but a male romanticism. I love these films, as they are so rare these days.