Its a good film. Not one as frequently great or "deep" as its fanboys like to pretend. Its certainly iconic, but if I bet if I pressed you to on the spot describe the characters to me beyond their physical characteristics, you'd struggle without running to google to find some shit that fanboys have filled in the blanks after the fact. Its cops and robbers done with a competent director and a great DP, but at the end of the day people remember that film because of a few iconic scenes and because of a gimmick (having de niro and pacino in the same scene) more than they do because it was well written or anything like that. (quite the opposite, Mann's dialogue is clunky. Pacino isn't roaring and chewing scenary because he's an indulgent actor, he's doing it to inject some life into the wooden script)
Mann's best film is definitely Collateral IMO which not coincidentally is also one of the few he got someone else to do the writing. Actually I think its the only film he directed that he didn't also write.