In terms of its plot, the film relies so heavily on the twist reveal that, knowing all the events and the outcome, it ends up being a thankless charade. Johnny Depp's performance, particularly the comic touch, saves it from being a total waste of time and the darker ending is a satisfying change but it's really not great.
I don't know what else to say. Part of me is lurching to come out and claim that if you've read a book you should never see the film because you know where the mystery tour is headed. The other part understands that you can know how a magician does a trick and still have your breath taken away by the grace and skill of its execution.
Here, it's just a case of "Oh, that's how it's done..."