We saw The Departed (**** Very Good) today. It's a Martin Scorsese film based on a Hong Kong movie, "Infernal Affairs".
The acting, especially Leonardo DiCaprio, is very good. The setting is Boston, so the actors all have Boston accents or try to have Boston accents. Matt Damon's accent from "Goodwill Hunting" has been dusted off and fits here very well. Jack Nicholson is a bad man who runs his little corner of the Mob with a heavy hand.
Matt Damon is a Mob guy who works for the Massachusetts State Police. Leonardo DiCaprio is an undercover cop who has infiltrated the Mob. During the coarse of the movie you can see each of them come apart from the strain of having two identities.
There are some unanswered questions that come to mind, so it looks like some plot twists are there for convenience instead of to serve the story. There are surprises right up to the very last second of the movie, and they aren't pleasant surprises. It's a Very Violent Movie (rated R for violence, language, and sex).
If you are in the right frame of mind, so that the violence won't bother you, then go see the movie. It's good.