I just saw this movie in theaters. This review is going to be kind of long, so let me put this in general terms:
If you're watching this movie because it has nice art: just watch the music video for the image song, Uchiage Hanabi by DAOKO. It contains almost all the nice shots from the movie (missing the ending sequence, which although is slightly odd, is very beautifully animated. So maybe watch the music video then wait for the movie to be available to stream then skip to the ending to see the pretty fireworks and surreal animation).
If you're watching this movie because it's Shaft or because of a famous voice actor: I don't think anything I say here will stop you from watching the movie, so go for it.
If you're watching this movie because you're intrigued by the story: just know that this 2-sentence blurb from Wikipedia (the one on MyAnimeList is very strangely worded and sounds very bad, honestly) is the entire plot: "The story takes place one day in summer. A group of young men are planning to watch fireworks from the town's lighthouse, wondering if fireworks are round or flat when seen from the side. Somewhere else, the class idol Nazuna asks the boy who likes her, Norimichi, to elope with her. What fate awaits these two in a day that keeps repeating itself?" That's it. Don't expect much more.
Art: This would have been the best part of the movie if not for the random CGI scenes. There were several minute-long sequences where the movie would switch between beautiful animation of the characters on bikes featured in the trailer to some horrible CGI rendering of the characters on bikes, then back and forth until they got to their destination. It honestly looked horrible and ruined the look of the movie. Other than that, the fireworks and other scenes were lit beautifully, and I can't remember there being any moments where the animation looked super rushed (though of course there were parts, but I count on stuff looking better on the Blu-Ray). Overall the art was cool to look at and Shaft definitely incorporated their own style by including their signature "head tilt" use of odd camera angles.
Overall it wasn't terrible, but definitely ruined by that horrible CGI.
Sound: This is what first drew me to the film and it was the best part of it, followed by the art (which would have tied if not for the horrible CGI parts). It was slightly disappointing that the image song only played during the credits, and there was one awkward part where Nazuna danced to a slow song for the full 4 minutes in the middle of the movie. While the song wasn't bad, its placement and length was; it made me feel like I was watching someone's ridiculous drug-fueled hallucination instead of Uchiage Hanabi.
The voice actors for the two main characters are actually live action stars rather than anime voice actors. I wouldn't have even known this if I didn't look it up. It was a bit odd to see famous voice actors play the side characters, but the only one that I noticed sounding off was Yuusuke's voice actor (who is an anime VA), who sounded really fake. There was a particular line when the boys arrived at the festival that sounded so bad both me and my friend turned to each other in confusion (we were wondering if he sounded that way on purpose to be sarcastic, but no, it was an actual line).
Characters: I know this is not an original work so maybe cutting some characters was out of the question, but everyone except for the main characters Nazuna and Narumichi, the male rival Yuusuke, and Nazuna's parents were unnecessary. Okay, since some of the other friends sparked the "Are fireworks round or flat?" debate that the movie is named after, I guess they are necessary too, but you don't need more than 2 of them.
While I've already stated they were unnecessary, there was a particular character that definitely should not have been included. The homeroom teacher (and her boyfriend, another teacher at the school) got some screen time in the beginning and a little at the end and I can't find anything that they add to the film other than to make those watching feel uncomfortable. The homeroom teacher is female and has a large chest, and there is a 2-minute long scene when the characters arrive at school of them guessing her bra size (one of the friends has a crush (if you can call it that?) on the teacher, which is what sparks the... discussion), then again while in homeroom the student makes a comment about the teacher's chest to her face and instead of getting reprimanded is slightly teased and the whole class laughs at his "joke." My friend and I found this to be very uncomfortable and it definitely made me feel worse about the movie. I can't remember the character's ages but they seemed to be in middle school if anything.
As for the main characters, they seemed to be pretty generic. Nazuna was a quiet "mysterious" girl that the two main boys, Narimichi (brown hair) and Yuusuke (black hair) have a crush on. I wasn't paying close enough attention so I'm not sure if they had a crush on her before a certain day or if they knew both of them had a crush on her. Narimichi seemed a little nicer and more awkward than Yuusuke.
Story: Honestly, the story blurb from Wikipedia was the entire movie. Other than that, I found a hard time understanding the motives behind the character's actions. I'm not entirely sure how to explain this without some slight spoilers for the beginning part, so for those curious I've put my explanation (skip the one paragraph if you don't want to read it) below:
I may have missed the explanation, but it is established that the character Yuusuke likes Nazuna. She asks him to go to the firework festival with her. He agrees, but then later blows her off. I'm sure there was some sort of explanation, but then later in the movie when Narimichi "redoes" the day and gets Nazuna to ask him out instead, Yuusuke flips out. I realize he has no knowledge in this redo that he was previously asked out, but his character should remain the same. Why did he freak out that Narimichi was asked out when he would have blown her off anyway? I'm sure there was some sort of explanation but it was quiet early in the movie and I must have missed it.
The story also suffered from poor writing at parts, whether it was from the original source or added in this adaptation I don't know. The movie also felt like it started to drag along towards the end when it lost its charm and it began to leap off the deep end. Two scenes that particularly stood out to be as bad are in the middle of the movie. The next two paragraphs are going to have SLIGHT spoilers, but I will describe the scenes in very general terms, so they're not huge spoilers.
Slight spoilers for this scene: Nazuna's parents figure out she is trying to run away and are dragging her away. Narumichi tries to help her and runs forward and grabs the father's arm yelling for him to let go of Nazuna. Instead of shaking off this tiny middle school kid like the large father he appears to be, he whips around and punches Narimichi in the face (yes, in the face, and Narimichi has bruises for a while). The boy then drops to the floor and the scene cuts 2 minutes in the future to show Nazuna's parents driving off with her in the car and Narumichi still on the floor. Wait, what? This dad just punched a kid. who immediately dropped to the floor and didn't appear to be able to get up, and left? What kind of world is this? The parents didn't seem THAT bad; definitely not perfect, but we've seen worse.
Another scene not far from the previous one is when Nazuna's parents are in the car driving on the highway next to the train she got on. The train exits a tunnel when Nazuna is standing in front of the window and people, including the parents in their car, see her standing and begin to frantically chase after her. Narimichi then redoes this scene and makes Nazuna sit down so that she is not seen. We are then shown a shot of her parents crying in the car and don't appear to be chasing the train as frantically. Why? They literally just saw her get on the train, and this is a rural town in the middle of the evening, it's not like they don't know which train she got on? The train also didn't stop so they actually should know for 100% certainty that she is still on it. But apparently since they can't see her in the window, they don't think she is on it. As a result, they aren't waiting for her at the next stop, which is what would make sense, right?
Overall enjoyment: The most subjective part of the rating. Honestly, for me this ties in very closely with the story, because if there are huge plot holes, I won't enjoy it. I can enjoy listening to the movie and looking at the pretty pictures, but I won't really enjoy the movie itself if there are some glaring flaws. Because of the terrible writing (in my opinion) and flow, I will have to rate my enjoyment and overall score a lot lower.
Final disclaimer: I am not a fluent or native Japanese speaker, but I was able to understand almost all that was being said, but I thought I would put this disclaimer here because as a non-native speaker, there are undoubtedly subtle things I missed.