Wavering between three and four stars, but ultimately it's a bit more towards "I liked it" than "I really liked it", so three it is.
I have a feeling this is one of those books where the verdict depends very much on what kind of a mood one is in when reading. Some other time, it wouldn't have worked for me (e.g. last year, when I bought it, and then realised I was rather overdosed on dystopian and/or post-apocalyptic YA fiction, if I'd actually tried reading it at the time, I'd have tired of it quickly, I suspect); today, it was just what I needed. I actually read the entire book in one day, which isn't something that happens a lot these days.
I sort of liked the world-building (although I'm not at all sure about the plausibility of the scientific explanations given for certain people dying and others changing and yet others surviving as they were) - plausible or not, it was internally (relatively) consistent and I've read and tolerated flimsier plot devices.
Characters ... now that was more of a problem. I actually liked Alex well enough, which helped, but most others fell flat, and the romance was rather cardboardy. Oh, and I could really have done without the triangle thing. Sigh.
The pacing worked great for me though and I was never bored, and that counts for quite a bit for me in books like this. I'm pretty sure I'm going to read at least the second book in the trilogy, too, seeing as I've had that one for a while already as well, but I'm not sure it'll happen any time soon.