My fellow reviewer is absolutely right- despite the visual appeal of this movie in which the style of fellow-countrymen Jean-Pierre Jeunet and Michel Gondry is heavily referenced, and the convincing portrayal of the trials of childhood love, when they reach adulthood you expect them to , well, you know...grow up, but no such luck- the restaurant "proposal" scene has to plumb new depths of psychological nastiness,and the viewer ends up completely alienated by their mutual petulance and cruelty.
Although there is some clever filming, overall I found this depressing. Two people pursue ever more incredible dares, destroying each other and those around them. The director mixes reality and imagination sometimes without any real clue asto which is which, to the point that the viewer can become completely confused. Sadly the message seems to be that there is no real meaning to life, apart from selfishness and thrills.