Matt Damon is unbelievably good. His character is about as horrible as it is possible to be; but he manages to keep you interested in his perspective and think about his motives. This film will haunt your dreams/nightmares but there is a strong and important moral lesson. The only criticism that occurred to me is that all the other characters are extremely privileged, but it is based on American young adults living in Naples and Rome. The plot would have had to be quite different to accommodate less wealthy characters.
As good a Patricia Highsmith crime novel adaptation as Hitchcock's "Strangers on a Train" or her sapphic novel adaption, Todd Haynes' "Carol".
The cast in this film is superb, [the late Phillip Seymour Hoffman's character is wonderfully odious] as are the twists, and the sense of time and place. So good is the sense of time and place, that the film really benefits from Blu-Ray and an ace TV.