The Departed is a remake of a movie called Mou Gaan Dou (or Wu Jian Dao in Mandarin Chinese). IMDB has it right here http://www.imdb.com/title/tt0338564/
I think Scorsese's "remix" of the movie left out a lot of the elements that made Mou Gaan Dou such a great movie.
The biggest complaint I get all the time is "everyone dies in the end". Actually if you understood the character Matt Damon was playing it would have been pretty obvious.
I watched both movies and I have to say that the original is better. It was very artistic, captured the essence of the city Hong Kong, with it's tall buildings of glass and steel, and the local people and life.
This is how I see it:
Matt Damon (forgot character name) didn't really have much of a choice when he was chosen to be a spy for the gangsters. After being a cop, he realizes he really wants to be a cop. This is why he kills Jack Nicholson and then the other mafia undercover - he wants to be a real cop and forget his past.
On the other hand Dicaprio *should* have shown signs that he is starting to become a real gangster, using excessive force and such, as seen in Mou Gaan Dou. This creates a role reversal, among many other things like the conflict between the two undercover people, the relationship between them and the psychologist etc.
Another thing is that Scorsese's award more or less was a "lifetime of achievement" award, rather than an award for "The Departed", seeing as how he narrowly missed so many oscars before.
Anyway I think the original is better with quite a lot of famous chinese actors. I think most asian people have seen the original already and were all a little outraged when nearly every single scene in The Departed was an exact copy of Mou Gaan Dou: death of police officer falling off the roof, Matt Damon with the envelope, the elevator deaths etc etc.
Open to discussion and welcome to add any additional info.