Saw it yesterday. Some good stuff and some weak stuff. I rated it a 6/10 on IMDb, fwiw.
Phoenix was very good but I do get a little tired of his "look at what I can do with my body" schtick. His ability to contort his body and willingness to hurt himself on camera is compelling but I don't think he needs it and it feels overkill at times. The rest of the performances are good, too, but this is very much Phoenix's movie. Like I said, he's very good but he's kind of playing Freddie Quell again.
It looks great. The first Oscars I'd nominate it for would be production design and cinematography.
The movie seems to confuse humorlessness with intelligence. It's so desperate to be taken seriously it doesn't bother to give much thought to the questions it raises. It wants to talk about how society deals with mental illness and then provides a lazy, one-note depiction of a man with mental illness. It wants to give the Joker a real-world origin story and then creates a character that, without the makeup, doesn't bare much of a resemblance to the Joker. It starts a class war but cops out by just demonizing both sides.
And, yes, I do think there is some recklessness in a movie about a troubled, lost man who finds meaning and community in murdering people. In and of itself, that's not a problem unless the movie validates the worldview. Which... it might.
If that's mostly negative, it's in response to what seems to be a largeley positive thread. I didn't hate the movie but I thought it dropped the ball on the most important stuff.