For a book I practically devoured, I really didn't love this. For starters, I have no idea why this is such a massive, worldwide hit. It's mystery does very little to set itself apart from any of a million other stories, and its payoff is incredibly weak. The characters, while interesting, just add up to the same problems of a billion other characters in similar stories (alcoholism, bad marriages, inability to have children, etc.). I felt like I was reading an amalgam of every mystery written in the past 10 years. 
 Also, it barely even lives up to its own premise. We've all ridden trains or buses past or driven or walked by a house and wondered what the lives were like of the people inside. It's incredibly human, and a great setup for the story. But pretty quickly we realize Rachel (the main character), has a mild connection to the people she's daydreaming about, which kind of ruins the mystique of her wondering about the lives of strangers. It just makes her a creep, and hard to really care about. 
 Pretty much everyone in this book is thoroughly broken. Which, fine, I guess. But I don't think people in general are collectively as broken as every single character is in this book, which makes it very hard to actually connect to anyone. I'd love it if, say, just a couple of people were broken? Instead, everyone is hiding a deep, dark secret that is just waiting to tear their entire world apart. I know I personally don't have a deep, dark secret that is just waiting to tear my world apart, and I don't think many of my friends do, either. Most people go their whole lives without a deep, dark secret tearing their world apart, so the pileup of deep, dark secrets here feels pretty unrealistic. 
 All that said, this book does read well. I flew through it. Paula Hawkins has a real knack for keeping things moving and nailing the pace of a gradually-unfurling mystery. I was so impressed with just the actual pacing itself that I might even read another of her books, even though I didn't love this one in the end. 
 Also, one last thing about the ending, so major, major spoilers: (view spoiler)[ 
 Why the hell did Tom suddenly start acting like a Batman villain at the end? This whole book, he's been shown to be nothing but a manipulative, calm, narcissist whose completely capable of making people do what he wants by telling very convincing lies. Then, all of a sudden, when he's accused of killing Rachel without any proof, he goes insane. He all but starts laughing maniacally. He starts yelling at Rachel and telling her how awful she is (something he's never done), and starts saying stuff like "You did this to yourself. Yes, yes. That's it! You're to blame!" He seems like he's steepling his fingers and twirling his mustache instead of calmly and methodically undermining Rachel and Anna the way he has successfully done for YEARS. I hated the way this was written. (hide spoiler)]