I'm writing this review upon one reading and before I've actually watched the film, and I can already say that this is a story that, as you get older and upon multiple readings/viewings, becomes more and more clear and real to you.
Story: 10
As a twentysomething who, by my own admission, feels lost in life, the story already extremely realistic in terms of the main message of the story. Life isn't a novel where at the end it says "and they lived happily ever after." It's one where people go in and out of your life, occasionally crossing paths with people from your past while meeting new people along the way. And along with that, there will be people you will meet and then will leave your life physically, but leave an impression that lasts with you forever.
This story isn't about two kids who fall in love as children and then find each other again and get married. This is a story that many people have experienced. One where you wonder "if this had/hadn't happened, could my life be different right now?" I know I myself have, and I'm not even especially old yet. For me, that made the epilogue (which I know isn't in the movie), especially relevant, as, without giving a spoiler, my own life is currently there and really made the ending hit home for me.

Art: 9
The art style is wonderfully done and is very clearly based off of the movie's animation style from I've seen. There's the occasional moment where the art style feels a little off for the tone of the story as a whole, but I don't think it ever gets too far off that it's an obstruction to the story.

Character: 10
The characters are wonderful. They're human. They feel like people likely have met in your life. They have real wants and needs. They have flaws and questions. They don't need some unique character trait to make them "different" or be trying to do something insane with their life. They are the type of person who exists in the real world. That is commendable, as, being someone who has tried writing short stories, creating realistic characters is not always the easiest thing to achieve.

Enjoyment: 8
Now, I only give this an 8 not because I didn't enjoy it, but because it is admittedly dry at times. That isn't to say it suffers there, but much like novels written in the 19th century like Jane Eyre or Wuthering Heights, it caters to a specific type of audience. If you're one who enjoys a story that doesn't need major drama or conflict to make its message apparent, you will enjoy this a lot. If you are expecting a conflict even in the range of Your Name (same author), you might come out a little underwhelmed, as this is much more of a realistic look at growing up.

Overall: 9
This is certainly a story that deserves to be read numerous times, as I can say upon even one reading that by the next time I read it, something in it will leap out to me as a thing I couldn't relate to during the reading I'm writing this review with, but do on the subsequent one. It's a book that, in a way, matures with you.