I had to give major props to Shutter Island. Not because of the twist, but because I had already read the book, knew what the twist was, and they still managed to surprise me. This is one case where the film actually improves if you've read the novel, because the ending totally screws with your expectations.
I don't want to say more, out of respect for people who've not read the novel or seen the movie, but let's just say the plot twist at the end of the film and the plot twist at the end of the book are not the same thing, and I actually like the way they ended the film better than way Dennis Lehane ended the original story.
Shutter Island, the film, was made completely on the assumption that you would figure out the major plot twist, either because you're cinematically savvy like that, or because you had read the novel. ESPECIALLY if you had read the novel. Because if you have, then DiCaprio's final interaction with the Shutter Island staff (especially Ruffalo) is impossible to see coming.
My wife and I loved the movie because it made this decision, and if you have not read the book, I highly recommend that you do so. You'll understand the twist the movie actually incorporated wasn't the 'big reveal' moment, but one little, seemingly-inconsequential detail at the end that DiCaprio sells like a champion and that I could not see anyone else but him pulling off. :)
awesome follow-up. Perhaps I will read the book based on what you have written here.
I don't intend for my opines to be the "end-all" for anyone reading them, it is just the way I feel about stuff. For example I absolutely love everything that Bruce Campbell is involved in although a vast majority of the general public would probably disagree with me :)