The "killer unicorns" premise didn't do much for me, so I was surprised to enjoy this as much as I did. I veered between liking and wanting to slap the protagonist, but I never doubted her authenticity as a real, live teenage girl. The secondary characters, sadly, are numerous enough that few are developed beyond a handful of key traits. I'm hoping the sequel(s?) will allow room for embellishment there, since some seem promising.
The romance subplot was thankfully free of tortured love triangles or excessive misunderstandings; whenever I found myself thinking, "You know, if you guys would just talk
...," the characters soon followed suit. The unicorn concept meant issues of sex (and virginity) came up frequently, and overall I thought they were handled well.
Handled less well was the relationship between Astrid and her mother, who seemed painted in black and white (mostly black) to remove any lingering audience doubts about whether Astrid was in the right in their ongoing disagreements. I would rather have seen a more sympathetic portrayal, which would have given emotional weight to Astrid's choices. But who knows: maybe she'll be fleshed out in the sequel as well.
Either way, I'll be reading it.