Light spoilers
This was objectively pretty bad, especially when compared to MCU1 films, in terms of plot, visual effects, fight sequences, and generally how the whole thing fits together. DC is trying to do something like what Marvel did in Spider-Man: No Way Home back in 2021, and clumsily introduces the multiverse to its cinematic universe.2 I don't really like how DC is just trying to emulate Marvel at this point, but it does make its recent films less awkwardly dark and more fun to watch.
The Flash follows a typical three-act structure (this is not a jab most of these films do). I really enjoyed the first act3 with Ezra Miller's younger Flash, laughed a lot. The second act is all over the place and the third4 is riddled with bad VFX and a clumsy conclusion.
5/10 (for the first act laughs) the Marvel Cinematic Universe ↩ this allows them to justify other versions of Superman and to introduce new alternative versions, e.g. starring Nicolas Cage, wonderful ↩ which goes until Zod's broadcast I'd say ↩ big final fight, obviously ↩Footnotes