I like this description Denby wrote in The New Yorker about Mann and his movies:

<Shot by shot, scene by scene, Mann, whose recent work includes “Heat” and “The Insider,” may be the best director in Hollywood. I don’t mean that he’s the greatest artist. He lacks such qualities as the tormented humanism of Scorsese, the generous showmanship and warmth of Spielberg, the moral curiosity of the Clint Eastwood who directed “Unforgiven” and “Mystic River.” But Mann has become a master builder of sequences, the opposite of the contemporary action directors who produce a brutally meaningless whirl of movement.>

The characterizations in this film are not its strongest point. I didn't leave the theater seriously contemplating the psyche of Tom Cruise's character. But, "Collateral", as directed by any other director, probably wouldn't be nearly as exhilerating or as interesting as it's turned out to be under Mann's direction.