Is it as good as "Gone Girl", or is it better than "Gone Girl"?
I've only watched Gone Girl, but Flynn was involved in the screenplay so it should play similar to the book. I think you'll be right at home in terms of Dark Places, she has a style that travels between books- like she is writing all of them at the same time. Very similar unraveling of plot and lapses of continuity as well as character forgetfulness for the plot.
You'll find characters who are similar to the ones in Gone Girl here and you won't like them. Personally I hated Gone Girl and this book I loved to a point. Don't expect it to better than GG, as people have walked away disappointed with that mind set.