Let me try to do this novel justice; I expect I won't quite manage it. To start, I can't recommend it enough.
The most impressive, skillful, and powerful part of Nothing Serious is the piercing clarity and accuracy with which emotions and sensations are depicted and described. As you read it, you come to understand loneliness, desperation, and isolation on a whole new level; alternatively, if you've had similar experiences, you see them illustrated with unbelievable, loving, painful attention to detail. If there's one thing the author does masterfully, it's this: She leaves you no doubt that she understands every character she's created, through and through.
Besides that, Nothing Serious also inserts and builds tension with surgical precision. This book contains abuse. A lot of it. Even so, as a reader, you're never quite sure when and how to expect it. There's an awful, horrible quiet that settles over the pages as you anticipate something happening; that turns into rising dread when you realize that an abusive character has been building up to something the whole scene, under the surface, and you're forced to watch the inevitable ensue. The novel captures and delivers for readers the horrible anxiety of living with an abusive loved one, and it does so tactfully and seriously.
On top of that, the plot does not drag at any point. Every scene offers something exciting or captivating. I won't spoil anything about the ending, but just...be prepared. Even though you can't be.
To offer a balanced perspective:
You may find yourself frustrated very often with the protagonists; they make frustrating decisions, as many people in bad situations do. While that's not a flaw of the work itself, it can affect the reader's experience for some.
I do also take issue with some of the side characters, who lack the incredible nuance of the major characters. Specifically, some of the side characters in Amber's story feel like means to an end as opposed to full individuals. They're only fleshed out insofar as it relates to Amber, which makes sense for a first person POV, but still makes them seem less real than their major counterparts.
TL;DR: Nothing Serious hits like a nuke in terms of depicting emotion and building tension throughout its riveting plot, even if it doesn't flesh out every single character or spare you the frustration of watching people destroy themselves. I can't recommend it enough.