Shutter Island is not a great film (i.e. it's nothing new or original), but it's a good film, which employs the elements of its genre well, and I enjoyed it quite a bit. But then, I've long been fascinated with mental illness, so just about any movie set in a mental institution would automatically pique my interest.
I would have to partially agree with some of the critiques--that because the primary protagonist (U.S. Marshal Teddy Daniels) is so closed and defensive a character, a certain emotional involvement is lacking. So I found myself impatient to get to the end.
But now that I know the ending, I think it would be even more fun to watch the film a second time. Although, unfortunately, knowing the ending is not sufficient to explain every detail of the plot. There are some scenes that don't make a whole lot of sense, even in retrospect. Some kind of commentary from the director would be helpful.
So, anyway, those are my thoughts ...