I read some other reviews and I have to say I completely dissagree with some of them.
Yes, even if it is about Cleo's and Frank's relationship, I think the other characters chapters add to the story.
I felt that the story unfolded pretty similarly to real life, where a relationship between two people is far more than that. (For example, whether you break up with your partner doesn't only evolve aroud your own feelings, it also depends on what your best friend will say about them, all your past relationships, how you grew up...)
I felt it was a fresh approach to the romance (?) genre, in how it subverted the almost sterile relationship between two people we've been reading about lately. 
Even if some of the characters are less likable, I felt I still cared about them and wanted to know more about their story. I finished this book quite easily and couldn't get enough of it.
It's not a 4 star book because I felt that some stories were cut short, right when something big/ interesting would happen.
Also, I think Cleo's character was very stereotypical (in a good way) of the "detached woman in her 20's" trope that we've been seeing lately ...
Lastly, congrats to Coco, they said it took them 7 years to finish this book. It's such a magical but also ordinary story on human relations and how each one of us is so different and special. Much love 𓆩♡𓆪