At the risk of over-generalizing, I wonder if this film would appeal more to European sensibilities than American ones. I notice that a lot of European films are more "slice of life" portrayals as opposed to traditional plot constructions (intro->buildup->conflict- >resolution). Yes, "A cambio de nada" has a plot and a conflict, but it feels secondary to the theme of the film - a portrayal of a young man's troubled life.
It's probably just my American sensibilities, but I was a bit bored by "A cambio de nada." Not that much happens during the movie, it's more just a description of a the troubled life of a Madrid teenager. But if you like "slice of life" type films, then you'll like this one. The acting is first rate. Of course, you would expect no less from Luis Tosar, 3-time Goya winner, who plays the troubled teen's father. But the novices really stand out here. The director's grandmother plays a lonely old woman who befriends Dario, the troubled youth. Even with zero acting experience, the lady is a star. Most of all, Miguel Herran, who plays Dario, is terrific. I can see why he won the Goya for best male newcomer ("mejor actor revelacion").
I'm glad I watched it. The quality of the dialogue and acting made the film more than worth my while. Even though I did find it a bit slow, I give it seven stars because the overall quality of the film is excellent, even if the film itself wasn't my particular taste.