Having said that, I also have to say the second half of the book was just as well written as the first half, but the plot veered too far into the implausible for my tastes. There were a couple things that pulled me out of the story:
1) There were too many writers in the book. Perhaps I'm biased since I'm a fiction writer myself, but one of the first lessons we're told is: Don't write about writers. Not only are Amy's parents writers, but Amy and Nick are both writers. Although Amy's parents being writers helped fully flesh out the plot, Nick certainly doesn't need to be one also. Because of the fake diary Amy plants as part of the evidence piling up against her husband, it might lend credibility for her to pull off the con by being a professional writer... then again, perhaps it's not necessary. Many people have very wonderful blogs nowadays, but aren't "professional" writers. And the fact that ALL the main characters share the same profession? Too many writers in the book, in my opinion. Like too many chefs spoil the broth.
2) Amy's psychosis reads too much to me like a made-up story rather than a fully realized narrative. She is logically crazy. I felt myself being stretched a little past credibility with some of the actions attributed to her.
3) Amy's character is inconsistent. She goes to such great lengths to protect herself and set up the narrative to frame Nick. Then once she stages her disappearance and reaches the cabins in the middle of nowhere, she immediately starts hanging out with two strangers whom she knows nothing about, even going so far as to reveal where she keeps her money and watching a TV show about her own disappearance with one of them. I understand that to forward the plot, her money needs to be stolen in order to be forced to turn to Desi... but it really seems out of character for her. It seems stupid and spur-of-the-moment, and the whole point of the book is she ISN'T stupid or spur-of-the-moment. She's insane, but she's methodical and patient.
4) Nick staying with Amy at the end of the story seems implausible to me. While the child might be SOME motivation, is it worth the rest of his life with crazy Amy? Seems out of character to me. He's turned to another woman before, and he could again. At the end, he talks about not wanting to become Amy, then contradicts himself by saying he's reaching her level of craziness. Again, this struck me as inconsistent.
***END SPOILER ALERT***
Other than these few plot points that bothered me as I read the book, I enjoyed it quite a bit. I'd definitely pick up another book by Gillian Flynn.