Not bad... I don't know how well it'll hold up over time, but I at least had a smile on my face by the end. The plot beats of the first thirty or so minutes felt a bit too obvious, and it's overall thin on plot, but it goes on a fairly entertaining streak after that. It doesn't really seem to have anything particularly deep to say and doesn't tie in to the Batman mythos in any sensical way, but it's worth seeing.
It's strange, I think there are two ways to view the movie: one is taking most everything that's happening as a reliable series of events (barring the one that's telegraphed) in which case I think the movie is pretty terrible. The second is viewing it as being fluid with time, space and memory. That the entire film is a sort of quilt of how the Joker constructs his self-identity, that HE is the unreliable narrator of the film. In that way I enjoyed it, where I think the only "literal" events in Joker are his mother's insanity, his own childhood abuse, his counselling and going off meds, and probably the murders on the subway. The rest to me felt like the Joker's own fever dream of a world overrun by clowns (projections of himself) and the Wayne family being a stand-in/loose connection for his personal obsession with Batman. Maybe I'm reaching or wishful thinking, sure, but it really felt like maybe The Joker wasn't actually/necessarily in this 1970's timeline, or at least his adult self. Him murdering DeNiro's character on the talk show being something he did much later and reincorporated into his past memories. And that the end of the film could have been one of the many times he breaks loose from Arkham, with Batman crouching on a rooftop, ready to swoop in and punch him in the face for the umpteenth time. Or maybe not, and that's just how I would have written it.
After thinking about it more, I believe the major thing that's missing was a convincing transformation... not just into "I'm not a dopey pushover anymore!" territory but into the realm "I've been consumed and my humanity is now gone". It doesn't seem like there's a line he fully crosses, like the filmmakers were too in love with the Arthur character or something. In order for this to work best the tragic aspect needed to really hit home. As it is, Authur remains a mostly sympathetic character throughout. At some point his arc needed to take him to a place where the audience can't follow anymore: either relatively gradually like Jake Gylenhaal's character in Nightcrawler, or just a point where he starts acting solely out of joyous malice in a manner that can't be justified by society's (or his mental faculties) abandonment of him.
What I feel is sorely missing from this portrayal of the Joker is his "self aware" side: the manipulator. Or at least the calculated psychopath side that feels purposeful behind all the anarchic chaos. Heath Ledger pulled this off well. He was silly at times, even kinda dopey, but could turn savage on a dime, and his overall scheming nature is apparent and ongoing.
Even if the Joker is intended to be the unreliable narrator, and sees his actions as motivated or even "good" in some warped way, there still needed to be a certain portion of the film grounded in "the reality" of the situation. Joker sees himself as a source of whimsical, (and murderous) attention seeking anarchy that exposes (what he views as the world's) hypocrisy; at some point the film needed to show us the monster. The fantasy/disconnect of Joker's warped mind versus the destructive consequences.
The Arthur whose humanity we connect with needed to be killed off, and it isn't, he just kinda settles permanently into the "redrum"/Private Pyle brooding "dark place". Sure, he seems to enjoy the consequences, but it doesn't ever feel like he has full agency, he merely dissolves under the weight of his horrible existence.