This was the best film I saw at London's 2004 Frightfest, much better than the over-hyped but ultimately disappointing Haute Tension, the other French language horror entry.
Superficially this is the Belgian take on the "crazed hillbilly" sub-genre of Last House on the Left or Deliverance, but in it's mixture of horror and surreal humor this is closer to something like Roman Polanski's The Tenant. The portrait of an isolated society who lives without women is taken to its logical and often shocking extremes. There is a scene at the local bar which must rank among the strangest and most memorable set pieces in recent years. The film is very atmospheric and the cinematography is stunning. You can almost feel the chill of the winter forest it takes place in.
Hopefully Calvaire (it's English title was The Ordeal when I saw it) will get a proper release in English speaking territories, though I can see that this is a much more difficult sell than the derivative calling card exercise that was Haute Tension.