Yes, Gone Girl gets off to a slow start, and the contrast between the diary entries by Amy and the diary like monologue of Nick left me with two thoughts: Nick is a total bounder and a cad, all of Amy's complaints and concerns are well founded, he 'did it"; and Amy is a sweetheart who deserves(d) so much better. And this comes with a sort of double distilled dose of antipathy for the guy and sympathy for the girl that is finally totally teeth on edge cloying.
But then, whamo!! I get further along and realize that although Nick is a jerk, his complaints are well founded, Amy is a first class way beyond the pale sociopath, the tenor of the book changes, becomes dark, turns into an edge of the seat thriller, and the badness on both sides is so bad it's over the top to the point of being hilarious.
Bottom line, stick with it, it was dark, funny, and entertaining.