I'm generally one who is excited over remakes, when its a proper quality production in the hands of competent people (and not jus typical recycled fluff, which its most of the time), and been several cases where I prefered the remake. But Departed was really, even if judged on its own alone, going for a big exciting angle of deep undercover in both police and gangster sphere WITHOUT actually bothering to build up the setting for it. It was good entertainment, but it was bit messy, didnt really go anywhere in what it was trying to tell.
Internal affairs was pretty on point on what it was trying to acheive and did it scene by scene. Quite tense in parts (police outside listening in, room full of gangsters and one mole, you can sense whats at stake with every shot), Departed missed that big time, it tried with the cinema meeting and cat and mouse chase afterwards, and how the Gang boss was eyeing the main character at times, but it really misfired in creating tension or build up the paranoia for those vital bits for sure. Matt Damons character in general showed the flaws of the movie in what it could have been, really poor use of that character.