Decent movie, good acting, not really a masterpiece. Technically there's nothing wrong with this movie. Phoenix's acting deserves all the praise, beautiful photography, etc.
From beginning to end, the movie is crowded with cinematic old clichés and rather cheap tear jerkers showing the downfall into madness of a very unfortunate man, which ultimately seems like a starved up pale attempt to pair itself as a film to brilliant pieces like The Shining, Taxi Driver or Cruise.
This movie really doesn't show any version I've known of The Joker, none. Since creator's Jerry Robinson's 1951 version up to Alan Moore's work for DC Comics in 1988, The Joker character developed in a certain direction that could have been amazing to explore in a more artistic way than previous DC films, instead this movie looks like the last assignment of an art student did before graduating, with enough money to hire DeNiro and Phoenix, but that's far from being such a breakthrough art film.