I'd give it a 3.5 were that available. Part of my difficulty is that one of my best friends loves this book SO MUCH that she got my expectations up, perhaps impossibly high. 
 I thought the world was really imaginative and mostly well thought-out (if kind of a stretch, as regards the genetically engineered beasties and their viability) (I was mostly willing to suspend disbelief, although the bats shitting steel spikes as a viable weapon made me roll my eyes). 
 I thought both main characters read as really young. They're supposed to be 16, but I kept thinking of them as 12 or so, and that was disconcerting. 
 Other than that, I don't know why I didn't like it better. It just didn't grab me that hard. I kept putting it down after a couple chapters to go do something else. 
 Edited to add: I went ahead and gave it another half star because my son! Ye gods, my SON loves this thing SO EFFING MUCH that he is making gene splicers out of Lego. I wish I were kidding. He wants to be a "boffin" and make his own crazy animals and OMG, Westerfeld, if my son gets busted for illegal and immoral genetic engineering, I'm blaming YOU.