Oh geez... This is a really hard book to review.
I'm honestly not sure what to think about Nightshade. I have no strong feelings toward it, and those books where you just don't give a shit about it, but didn't hate it are the hardest to review.
It wasn't completely terrible. The writing is decent, no grammatical errors (as far as I could tell), and there's no awkward phrasing or anything. I did like Calla for the most part; she was nice enough. Shay, in my opinion, is kind of like Peeta from the Hunger Games: sweet but completely useless, and therefore sometimes annoying. I didn't like how Shay insulted the traditions of the Guardians, in other words, Calla's traditions. And I didn't like how Calla turned her back on everything she was raised on just for some random guy.
I hated Fey and Dax. Those two were so disrespectful to not only Calla, who is their superior and their leader, but to Mason and Neville. Come on, really? How is it that in this day and age, there are people still against same-sex relationships? These neanderthals need to evolve or something. It's 2014. Get with it.
Ren....He was the most likable character, besides Calla. In the love triangle, I like him better, because he's sort of a bad boy, but not a jerk. Mostly. He's way better than Shay, I'll tell you that.
Overall...I don't think I'll read the sequel, but I do recommend Nightshade. If I had to describe Nightshade in one word, it would be: Adequate. Average. Decent. Not horrible, but not amazing.