I'm afraid I didn't have the time to do a full review, so here goes:
Martin Scorcese's remake of the Hong Kong industry's Infernal Affairs is riveting, and is a reminder of an era when massy, commercial films and intelligent cinema weren't mutually exclusive concepts. While the evocation of the moral ambiguity, indeed the frankly compromised nature of the state's attempts to carry out its police functions, is a staple of intelligent noir, Scorsese lends it great weight and purpose, and ensures that viewers experience this moral ambiguity almost as an operative condition of being. Clarity? Clean dividing lines between good and bad, law and criminality? Not in this world, baby. The casting is inspired, from Jack Nicholson's larger-than-life barnacle, sordid through and through, to Leonardo DiCaprio's one note existensial anguish (but what an effective note it is!), and the smooth-faced Matt Damon (whose physicality goes well with... ... his character, that is to say not so much "bad" as morally blank), not to mention the likes of Martin Sheen and Mark Wahlberg, the film is effectively acted at all points.
Scorcese the director is in total control here, and after his uneven outings in Casino and Gangs of New York, this film might be a reminder that he is strongest when engaging with a human story, not with epic sweep; in particular, stories of people at the mercy of forces larger than them. That is true in some sense of every character in The Departed: one lives only dimly aware of the "real" stakes, and any insight is itself simply a harbinger of death, almost as if death were the price exacted for knowledge. That is to say: one will never know, until its too late for the knowledge to do any good.