Saw it yesterday and had some time to sleep on it.
The characters were great - that they managed to make Aquaman not suck is quite a feat. The "talk to fishes" jabs were great, and Mamoa did a terrific job of making Aquaman feel imposing (and some credit here actually to Snyder too, for deciding to have a real feeling Aquaman, rather than a so-Cal surfer bro). The movie was overall enjoyable.
Flash as has been mentioned was great and stole his scenes.
I think this was said before, but Aquaman and Flash should have had their origins prior to this. I liked the backstory of each, as forced as they were, and wished we would have seen more of those stories. The Atlantis scene was pretty cool and they captured the underwater effects well, and the Billy Crudup scenes in the jail gave a nice bit of layering to Flash that we should have gotten in its own film.
Giving them origins prior to this would have also allowed for a more epic final battle with a reunification of the forces of Atlantis and Thymiscera to combat Steppenwolf and his parademons, thus also making Steppenwolf more intimidating. Plus, it could have allowed this film to really focus on being an origin story for Cyborg and being told from his perspective (much as Avengers 2 was told from Clint Barton's perspective).
The biggest flaw for me (in that it took me out of the film more than anything) was the art-direction/post-photography-effects (coloring and feel) being... just not quality or consistent. There were some good stylistic choices here and there, but likely due to the time constraints of reshoots (how there were time constraints with such a lead up is a joke), the film had this Sky Captain feel with the green screen feeling apparent, rather than used as a tool to make it feel comic booky (which I think is what they were going for?).
Ohh, and those wigs made the reshoot scenes so glaring.
Well, I guess here's to hoping for a Flashpoint reboot where they keep Gal Gadot, Ezra Miller, and Jason Momoa. Everyone else they could recast. Though I liked Affleck as Batman, he gone. I also liked Cavil, but they f-ed up Superman too bad to bring him back (to not fault of Cavil, IMO).