This is one of those books where the set-up had me really nervous: too contrived and implausible, the kind of thing that Hollywood loves for its rom-coms, featuring characters who never behave like actual human beings. But I am learning not to underestimate K. A. Mitchell, because both Nate and Kellan are two of my favorites of her characters. I have a soft spot for couples who have known each other forever, and you believe it with these two. I also liked that they were seriously flawed but not to the point that I hated them. I spent the first half of the book worrying that Kellan was too big an asshole to be redeemed and Nate was too completely superior, and then halfway through the balance shifted. Kellan was a sweet-heart and Nate was never going to get the stick out of his ass to have a relationship. All of this is just to say I was invested. I loved seeing Kellan turn into an actual worthwhile human being, I found Nate's hyper-complicated self-righteousness endearing.
Minor Spoiler: I'd also praise Mitchell for not turning the dad into a caricatured villain. I generally find them tiresome in and of themselves, in addition to being unrealistic and overly convenient. The father wasn't a good guy, but he wasn't all things that are evil and hateful either. It fit with the story well since it belied Nate's black and white attitude, but it also made the story more realistic and satisfying.
Altogether a great read--I've already loaded the second book of the series on my Kindle.