Let me first say that, overall, I enjoyed this book, though I do have some mixed feelings about it. More on that later. I shall first say what is good about it.
I could just say that the writing is good, and though true, I must also say that I loved the ambiguity of the setting (I get the impression that this isn't Earth), I loved the occasional Biblical symbolism, I loved the imagery, and I loved the technology present. Though nanites aren't the freshest of sci-fi tropes, Picard still makes them magical, and all the other technology is either scientifically plausible or awe-inspiring, and frequently both. Zombies are pretty done-to-death in this day and age, too, but Picard's zombies can be alternatively funny (they really seem to like lawn flamingoes, for some reason) or disturbing. They never really pull off "intimidating," but that role falls instead to the villain.
With that, I must discuss the characters. The villain is great. He's exactly how I'd expect a HUGE NERD to act upon discovering super powers, while simultaneously coming across as evil and insane from casually causing wanton death and destruction, mostly just because he can. The main characters, however, are the source of my mixed feelings. The main protagonist, Regan, is a lesbian with a huge crush on the other main protagonist, Alisia. Regan survived the first zombie-outbreak, and Alisia is a soldier, the first normal human Regan saw after two years living in the zombie-infested ruins. The problem is, Alisia is straight, and Regan is extremely forward with her advances. So forward, in fact, her advances would probably count as sexual assault.
There were times in this book that I thought the overt sexuality was about as tasteless as one of those Japanese harem animes, but then we'd get a look at some of the motivations behind this, and we see that Regan's not quite right. After losing her brother and living alone amongst zombies for two years, she bears some significant emotional issues. There were scenes involving her emotional frailty that were heartbreaking in their poignance. Over all, I suppose the two extremes kind of balance each other out, but it still gave me a nagging feeling at the back of my head.
One last nitpick: while the pacing is generally good, there were a few slow spots. Particularly, the middle part seemed a bit lacking. That's usually where I'd expect a major disaster, but what we got instead was sort of a minor victory and a really quiet disaster. Oh well. Things would always pick up sufficiently later, making for an over all satisfying read.