Sam Mazur is literally me as a person. My favorite character in this book. I thoroughly enjoyed the middle, about the second 3rd of the book. The beginning and end were slow. After I finished reading, I was still confused about the plot or point of the book. There was barely any character development for Sadie, except for some stuff thrown at the end. Their "conflict" as a friendship seemed one-sided and shallow.
It was still an enjoyable read and hard to put down. Maybe it's just to tell the story of one friendship but it seemed like most of the time they were being spiteful and bad at communicating than they were being the best of friends. It's hard to understand why they even are good friends, especially because their relationship doesn't seem to develop more deeply after their childhood. Once they reunite in college, they seem to be more colleagues who have great chemistry than lifelong friends. I just wish the author had delved a lot more into their friendship (and gave us more adult examples of their very deep friendship without work) before spending the rest of the book with them being spiteful towards each other for somewhat idiotic reasons and assumptions (seriously Sadie's reason for avoiding Sam is based completely on assumptions that aren't true whatsoever ever). 
I still loved some of the characters like Sam, Marx, and Zoe but god, I despised Sadie, she pissed me off for the majority of the book (Selfish, Self-Pitying,  Naïve, ignorantly privileged and so on). 
Also, the dialogue was very clunky and at times seemed unrealistic. Like I understand that the characters are supposed to be smart but I doubt that their intelligence causes all three of the main characters to talk in the same, weird way.
Still an enjoyable read but I was definitely able to find some flaws which I usually don't do while reading the book for the first time so it somewhat annoyed me.