Utterly dreadful. We watched this so we could understand the events of the next film, but if you're wondering whether to do the same, I'd recommend not wasting these two-plus hours of your life and just checking out a Wiki plot summary instead.
I liked the first one, despite the facile plot development and the loose use of magic to do literally anything that plot required - the characters and the monsters were charming and smoothly introduced, and the story had genuine fairy-tale excitement - but Crimes of Grindelwald was a muddy and incomprehensible mess. I gave up halfway through the first time I watched it, when it became clear that most of the actual story was being covered in a ten-minute-long, joyless flash-back exposition (from a character who was entirely unnecessary to the film, who was mostly talking about characters we'd been given no reason to care about). The plot was meaningless drivel, and the characters were largely absent.
Newt seemed able, with his menagerie of biddable monsters seemed able to accomplish anything he wanted in the first film, but that seems to have been forgotten now; he is reduced to a sidelined spectator, who has apparently forgotten about what he's got in his case. The plot also performs cartwheels to explain and justify the presence in Paris of Jacob, Tina, and Queenie - but why is not clear, because none of them really have a part to play in the story.
Much has been said about the young Dumbledore too, and there should be nothing wrong with Jude Law in that role - but again, he doesn't really DO much (except some more flashbacks that also fail to advance the story), and it was continually bothersome that he had a completely different way of speaking to the older Dumbledore we know from the Potter films - different accent, different cadences; just clearly not the same man. Why wouldn't he play the part of Dumbledore, instead of playing the part of Jude Law?
Depp as Grindelwald has menacing presence, and is the only character given any significant screen time, but after the first (admittedly thrilling) scenes featuring his escape from imprisonment, he doesn't really DO much to flesh out this menace. The story just doesn't seem to move forwards, or make sense - which is fine if it is fun and/or appealing in any way but it's not. And the film's climax, featuring a speech by Grindelwald that has no apparent purpose, and then a circle of blue fire which for some reason at some arbitrary point becomes a huge bird-shaped blue fire apparently about to destroy all of Paris for no clear reason - well. It was probably the most disappointing and empty end to any film I've managed to watch to conclusion.
It's difficult to find anything positive to say about it, except that I do still want to watch Newt more, and a proper battle between Depp and some monsters would still make a decent film. I hope to Grindelwald that the other ones are better than this.