4.5 stars. I had to wait for 4 months to finally be able to borrow it from the library (10 people ahead of me, bah), and I admit I was worried about the whole epistolary/chats/format thing, because it's too easy to just slap "witty" formatting on pages and call that a good book. Fortunately, the story and characters really hooked me--yes, even though the "chat-speak" on some pages would normally quickly have annoyed me in other circumstances.
There was romance, but not of the stupid kind that casts a shadow on the whole plot. While Ezra and Kady do think about each other, after their world was turned upside down and forced them to reconsider the importance of a breakup for what I shall call "puny teenager reasons", they didn't spend their time mooning, groping each other, and sending the rest of their crews to hell just because feeeeeeelings. Contrary to what happens too often in many YA novels (you know, "the world is ending but we're still too busy pondering who we want to go out with"). It wasn't the best romance ever, but at least it was far from being the worst I read.
The format: fun to read, although I tended to prefer some aspects over others (the "chatspeak" conversations, for instance, weren't my favourites). I think this was because of the whole "report" side of it, which made for a strange dichotomy in the narrative: both very factual ("here are recordings from camera X") and unreliable (how were those recordings recovered and put back together?).
The characters: so I'll be honest, Ezra and Kady were "good" to read about but not really more. Ezra enlisted to help, not knowing much to what he was signing up for, yet aware someone should help, and gradually revealing a few things about his family that were hard to suspect at first. I didn't care for Kady much at first, because of her sullen, do-not-want-to-do-that attitude; however, she redeemed herself later, and anyway was more of an action girl with a bit of romance on the side than the usual (and annoying) lovestruck heroine who lets The Boy do all the work.
Yet as far as I'm concerned, the one character I really, really loved reading about was AIDAN. It's crazy and damaged and desperately trying to latch upon what's left of its programming, pushing its core directives in directions that may or may not be completely at odds with what the crews of the three spaceships would like (or need). And in doing so, in being so, it was all so human, spiralling towards both destruction and a new awareness.
And, frankly, I just loved that AI. It added at least a full star to my rating. No kidding. And just for AIDAN, I'd read it again. (Also for the unreliable parts I mentioned above, and the way they tie together, and you'd better pay attention to a few tiny details here and there.)