I'm a Lars Von Trier fan. I love most of what he does and this film is unmistakably his work: the usual handheld cameras, a disfunctional main character caught in a difficult situation, some surreal elements (in this case a previously unknown planet called Melancholia appears from behind the sun), and some subtle humour (the wedding planner who refuses to look at the bride).
The cast is brilliant, Kirsten Dunst in particular but also Charlotte Gainsbourg and Kiefer Sutherland.
The weakness for me is partly in the disjointed nature of the plot, which is in 2 parts that don't fit well together and in the conclusion, or at least in the meaning of the conclusion, which is left (I assume) deliberately ambiguous. There is more than enough room for the viewer to add his/her own meanings to the planet, it's effects on the characters and so on. While I enjoy some freedom to add my own meanings, for me there are too many loose ends which is unsatisfying and a bit of a cop out as if Von Trier couldn't quite decide what it all meant and so left it up to us.
With that said, it's interesting, unusual, well acted and worth watching but not a classic.