Schematic but tense and quite sad story of alienation involving a cop who goes undercover in the mob and the gang mole who infiltrates the police. Strong performances by Tony Leung and Andy Lau as the two men living a decade out in the cold. One of the better gangster film of the century.
It certainly had it's moments which is why I would probably give it an 8/10 (4/5 on here) it's interesting how it came to influence The Departed so much but in it's own being a good movie and having many memorable moments, the performances like Steve said are great and one of the reasons why this film is so good.
I can't really decide what was wrong with this film, it was quite well done but after falling asleep twice in it I would just class it as uninspiring.
What is good about it however is the soundtrack (although the music is a little odd), there is an uncompressed PCM 5.1 option which is stunning and ranks amongst the best I've heard.
Turn it on and listen loud, it may keep you awake!