First Half Was Terrible, Second-Half Was Awesome; Iris Was Miscast
The first 15 minutes was an absolute nonsensical mess that never should have been in the film. I guess they wanted the whole Justice League thing in there so they contrived the most ridiculous, over-the-top, asinine "hero" sequence so they could toss Gal Gadot, Batfleck, and some Flash special effects in there?
Also, Iris was a fat black wench... like seriously? Funny that they had to keep making her ask out and pursue Barry, which kind of made sense given that even as a dorky, obnoxious, annoying dweeb he was still leagues above her.
All that being said, Ezra Miller really hammered in his role by the second half of the film. In fact, the entire second half of the film was really, really, really well done with the time paradox, Barry realising his mistakes, and trying to just let things go.
The best acted moment out of the entire film, though, was prime Barry berating the younger Barry about being immature. The other sequence where prime Barry is trying to reason with younger Barry about losing his mom and his world was also good, but they really should have gone a step further in talking about the moral juxtaposition of prime Barry getting to go home and younger Barry losing his world.
In some ways, this film could have been so much better than what it was, but it felt like there was so much studio meddling just ruining the flow of the film.
They should have extricated the entire first 15 minute action sequence, it was pointless and obnoxious and it wasn't even fun to watch. The Batman stuff with Michael Keaton was all on-point and well done. I don't think anything should have changed with that.
They should have done just a tad more about the paradox of one Barry getting to live out his life and the other not. The end "boss fight", so to speak, was really rushed when that is what should have been more of the focus. Also, it should have given younger Barry a bit more of an impetus to interfere so as not to become like the phantom zone Barry.