I'm usually pretty bad at figuring out whodunnits, but honestly I solved the Katy Devlin murder at around the halfway point -- it was just too obvious. That was a major failure of the book made worse when Ryan addresses the reader at the end and suggests that we have been just as befuddled as he was. French perplexingly seems to suggest that she's pulled a "Murder of Roger Ackroyd" on us with a narrator who tells us in the very first pages that he lies. But in fact he hasn't lied, not even by omission; he's just been phenomenally stupid.
The second major failure of the book was in the way French crafts characters and relationships. The cutesy-poo banter between Cassie and Rob might be fun for a 16-year-old to read, but I found it boring, annoying, excessive and hugely unrealistic. Every single time they interact, there has to be an exchange that I guess the reader is supposed to find clever and sexy, but in fact, the playfulness of their relationship struck me as a kind of clicheed teenage romantic fantasy: the guy and girl are best friends (though not lovers -- yet) and everyone believes they're in love but they are the last to realize it themselves; then when they finally do sleep together, it changes everything...oh please, Ms. French; save that for your YA book.
Moreover, French seems to like the character of Cassie so much that she makes her just about perfect. Cassie is always right, and she does almost all of the detective work on the case. Rob does end up making a key breakthrough, but does so in a way that seems like a fluke on his part, plus that's his sole contribution; everything else is done by Cassie, who is also apparently the only person on the force who knows the definition of a psychopath and understands profiling. The result of this is that, ironically, after a while I started to wonder why we even needed Rob in the story at all. I also think this is part of the reason why many readers found Rob unlikable -- Cassie is so flawless that we can't help but see Rob as excessively flawed, which I'm sure is not quite what French intended.
And of course, there's the ending. I am not against ambiguity; in fact, many contemporary mystery novels leave at least some part of their plots unresolved as a way of adding realism; no matter how much we may want to seek the truth, a detective knows better than anyone how impossible it is to find it absolutely. And yet, as others have said, the ambiguity here serves absolutely no purpose (except, as has been suggested, to pave the way for a series). If the idea is supposed to be that "some things simply can't be uncovered," we hardly needed 400+ pages to understand that. Moreover, in these 400+ pages we learn almost nothing new about the 1984 case other than a few vague hints of what seems like supernatural forces -- and, importantly, Rob doesn't seem to have learned anything or changed at all after going back to the woods. Why even bother writing about it then?
On the plus side, yes, she can write beautifully at times, as many have said. But frankly I'm getting a little tired of all these super-mega-best-sellers covered with glowing accolades that make it seem like you have to read it or you'll be missing out on the event of a lifetime. I see it more like this: if you read this book, you'll probably find some of it quite enthralling but a lot of it disappointing; if you don't read it, don't worry about it too much.