Like everyone else, I've heard a lot of buzz about this book. Entertainment Weekly has a cover featuring the stars of the upcoming movie adaptation, Rosamund Pike and Ben Affleck, and I figured I'd better read this before film gossip might ruin any plot twists for me.
I would suggest you do the same, as there are many twists in this book.
This book, while intense, is not without faults. There are odd metaphors ("He swallowed his lips" stood out for me as the worst) and repetition of adjectives (items are described as "creamy" about eight times). Luckily these faults don't detract from the overall story, and its powerful effect on the reader.
Often I can guess a plot twist, and I did do that on this one, but even as a huge twist is revealed in the middle of the book, there are more coming. It's staggering, and the ending is so dark that I think Shirley Jackson would have grinned in delight at the sheer nihilism of it.
Stay dark, my friends.