The only bad thing I can find about the movie is that the movie is the remake of a 2002 Hong Kong movie named "Infernal Affairs". This means that the script is not original but still the cast is so good that it makes you feel like you're watching a totally different movie owing to the actors/actresses's great acting talents.
The movie takes place in Boston, US and is based on two "rats". The Irish mob boss, Frank Costello (Jack Nicholson), plants the boy he's been mentoring for years, Colin Sullivan (Matt Damon), within the Massachussetts State Police as an informant. At the same time, the state police assign undercover cop William Costigan (Leonardo DiCaprio) to move into Costello's crew. When both sides realize the existence of the informants, each "rat" tries to find out who the other "rat" is...
Besides these, Mark Wahlberg, Alec Baldwin and Martin Sheen also have fairly major roles in the movie...
Although the movie had been a remake, it managed to win 4 Oscars including the Best Picture and the Best Director, which was the first Oscar of Scorsese throughout his whole life.
According to authorities, including the Rolling Stone magazine, "The Departed" was the best movie of the year 2006.
In conclusion, if you want to see a crime masterpiece with an original script and loads of talented actors, you'd better watch this as soon as possible!
Here is a very good scene from the movie, enjoy!