Sitting in the cinema on Tuesday night waiting for the film to start, I was absolutely petrified. I only went because my boyfriend wanted to see it, and my dread became even worse when I discovered it was 18 rated on my way into the cinema.
I've only ever seen one 18 in my life (and it was American Pie Beta House - which I don't think counts!)
BUT - the film was not as scary as I had anticipated. It was good though. It kept us both gripped, and the two and a half hours flew by. Ben Affleck played Nick brilliantly. His performance was only flawed by the early violent scene where it appeared completely out of character - I didn't believe in him. Rosamund Pike on the other hand, was consistently believable as 'Amazing Amy' - a woman haunted by her idolised childhood. It seems to me she staged the whole thing because she's never been able to compete with Amazing Amy, and this is the first time she could. Only Amazing Amy could come back from the dead.
My main disappointment with the film is my own fault really. I had read the book during a summer where I was reading a couple of books a day. One of these books was Lucie Whitehouse's Before We Met, which has been praised as being the Brit's answer to Gone Girl. The ending of Gone Girl was much more muted than I was expecting due to this confusion.
So if you were also left a bit flat by the ending - I highly recommend reading Whitehouse's novel. I guarantee you'll be satisfied, and I hope somebody picks up the rights.