I was so afraid to watch this, since I am sensitive to gore and horror films, however I wanted to see this Lars von Trier film so badly based on his prior films. Fortunately, it was not as bad as I feared... but be warned, there ARE extremely graphic scenes that many people will NOT want to see.
I read the Wikipedia page and watched relatively tame clips on YouTube... so I was prepared to cover my eyes during the scary and gross parts. Also, I did not watch this alone but rather with someone able to withstand what I cannot since to him it all "looks fake". To me, the special effects in a well made movie set my anxiety off all too well.
Those aside, the film is exactly as one would expect with von Trier... philosophical and psychological analysis of larger human truths through the lens of the actions of the characters... and that means through the thoughts of the director.
His comments about Hitler at Cannes some years ago are echoed again in a discussion between the two main characters later in the film. "The Value of Icons". Absolutely disturbing content and abhorrent actions and thoughts are contained in this film, as they are in humans. He forces us to acknowledge this and wrestle with the fact that we aren't so lucky as to be able to discern between good and evil and that instead we all live on a continuum.
And one more thing I MUST mention, Matt Dillon is incredible in this. Although his character is not consistent with what we know about most serial killers (such as having a usual Modus Operandi for example), he is able to bring to life this psychopath in such a way that you really fear for everyone he encounters. Excellent acting from him and the rest of the cast. Despite the brash subject matter, the director is able to gather a stellar set of professionals. No small feat.