Since I had heard that there was a twist, I tried to guess what it was when I started watching the movie. Turns out, I had it pegged by the time he got on the island.
I thought it was pretty good, not great. I don't know how all of you saw the twist at the end coming, but it surprised me.
I, and others I've spoke to, figured it out in the first 10 minutes when the "partner" has trouble with his gun and they make a big point to show it and everyone's faces.Don't read this if you haven't seen the film yet. I did not like this film because it messes with the audience's head and jerks you around. First, you identify with the main character and his partner as all the evidence seems to point in their favor; that is, the island is run by creepy sadomasochistic shrinks, the warden is a crazed ex-military man, etc. Plus the flashbacks to the DiCaprio character's wartime experience invite you to link the Shutter prison with Nazi concentration camps.
Then you learn at the end that the DiCaprio character has been a patient there for two years, killed his kids (when the film earlier suggests his wife did that and other possibilities), and will now be lobotomized because the alternative "therapy" did not work. I simply do not care for such manipulation of the audience; it was as if we watching the film were deceived by the director just like the DiCaprio character was deceived by the asylum administration. A cheap trick, really, in my view. Guess I prefer straight realism.