The first 20 minutes is pure pornography. It is not done distastefully but it is still pure pornography. While that scene is essential to the entire story, it could have been shortened to a lesser 5 or 10 minutes of lesser explicit sex without jeopardizing the story line. But if you are turned on by gay orgies then you have nothing to complain.
But it is after that marathon sex scene that the film took on a completely substantial value. The performance by the two young actors is good but what makes this a masterpiece is the overall story line and flow. It is a real-time film, meaning the time frame of the story being depicted is the real time frame of the length of the film. It really draws you in - if you allow it to. In typical French fashion, it's the undercurrent tension that grips you rather than anything in-your-face. On a deeper level it reminds me a little of Crime and Punishment (Dostoyevsky), not in the nature of the plot but in the subtle yet strong alternating waves of emotions: between morbid fear and banal carefreeness; between romance and anger; between naive innocence and bitter reality; between hope and despair, all happening with the dark, ordinary yet enigmatically charming Paris, as the stage (you won't see any glimpse of the Eiffel Tower, nor the Sacre Coeur nor the dirty ghettos - you see the real Paris ordinaire). It is a plot that lends itself perfectly to French cinematography and style; a story that screams to be given the very French treatment of film making.