I thought it was fine, a 7 out of 10.
I feel the Internet hype and clickbait culture has let it down - many film critics have gone into it thinking it is politically dangerous, which it isn't - if any viewer finds the main character to be inspirational then they are as deluded as him. Many comic-book fans have gone in expecting a all-time great film.
It is basically a story of a very damaged man, that loses complete touch with reality, loses empathy, and does terrible things to others. It is a fairly slight story really, although I think the plot works quite well.
When you compare it to the films it emulates - like Taxi Driver - I don't think it is as good as those. Taxi Driver was more powerful because in that film Travis Bickle is an almost ordinary man that takes extreme actions, whereas Arthur in Joker is mentally ill from the start.
Thinking about it later an idea I took out of it was how mobs and revolutionaries can too easily come to believe a person represents something that person really isn't.
Another thing I thought when watching was the trailers showed a lot of the film, without giving away the plot necessarily.
A lot of it is well done - like the last 30 minutes is very tense. It certainly stayed in my mind for a while after watching it.
Oh, and by the way, there was one song-choice I was quite taken aback to hear used - it's as if the film-makers didn't know what they were using there.