Reading the manga, a lot of stuff bugged me. Mostly how quickly the storyline just moved on. The characters and relationship didn't get time to properly develop or even be introduced well. The idea of this story was cute, but the way it was told turned the manga pretty shitty. However, in the movie they changed all of that. The creators stretched out all of the "soon-to-be-over" scenes, and ended up spending their time very well. This way it was easier for me to really get to know the characters, and for their relationship to build. They also managed to not stretch anything too far. They changed some of the moments in this story from being weird or feeling unnecessary to being absolutely adorable.
The artstyle is unique and simpler than what we're used to, I really liked it. It's very similar to the manga artist's artsyle. Animation is smooth and fits this movie perfectly. Just the animation itself made me laugh a several times. The backgrounds are also gorgeous. I loved all of the usage of colors.
The soundtrack somehow gave Doukyuusei a realistic vibe, and Galileo Galilei's song playing at the ending was perfect.
Both Kusakabe and Sajou are mostly chill characters, and seeing them get along like this makes me very happy. Their love doesn't seem forced, which is in my opinion the most important thing when it comes to love stories. Sajou is a typical nerd/careful character, but something about him makes him stand out, as if there's something that he really wants. Kusakabe is not like the typical rock star/talk-without-thinking character. He's calm and very nice. They are both just high school students, which is very easy to notice. They make dumb choices like every teenager does. I love how their ages actually show through their personalities.
I would recommend this movie to anyone, whether they like shounen-ai or not. This is what a good love story looks like: characters falling in love the most natural way.
The only thing I didn't like was one of the changes from the manga. I wish that they would tell us something about Sajou earlier. ***SPOILER He has a fear of trains SPOILER IS OVER*** In the manga, they made this very clear, and it cleared up what was going on in some of the scenes. In the anime, this didn't come through well at all, and has definitely caused confusions.