Ok, finally got round to this. Saw the extended cut, which was probably a mistake because 3 hours of this was really trying.
Anyway... it's bad. But it's fascinatingly bad, like an unpleasant fever-dream that constantly moves too fast for you to get a grip of. For at least the first hour, probably longer, we plunge in and out of scenes with barely any sense of where we are, what characters want, and what the point of it all is. It really does feel like you're watching the movie on a bad drug trip. If they took up all of the movie's first half scenes and shuffled them like a deck of cards to create a totally different order I really don't think it would make much difference.
I don't hold anything against the actors, they did the best they could with the material. I even find myself mildly curious to see what Snyder will do with Justice League, because he's clearly gone beyond the point of taking in any of the many criticisms leveled at him. It will be his hellish vision.