Great hair … but why keep the stars parted too?

Though it has moments of brilliance, American Gangster dazzles with a promise it doesn't keep. Between Scott, the director of Alien, Blade Runner and Gladiator, Schindler's List scriptwriter Steven Zaillian, Washington and Crowe, there are enough statuettes and gongs to melt down and make a life-sized model of Cedric the Entertainer (be kind - it's the closest he's ever going to get to an acting award). There's a great, true tale to tell here of a drugs baron pursued by a lawman. And yet, and yet.

The biggest problem is easy to identify: it's Scott's failure to pass what might be called the King Kong versus T Rex test, named after the iconic moment when the big ape takes on the toothy lizard. In Washington and Crowe, Scott has two of the biggest beasts working in movies today. They have shared a screen before, in the sci-fi thriller Virtuosity, but that was 12 years ago, before Crowe was the A-lister he is today. Now they're of equal rank, the audience might expect to see them tearing into each other from the off. Yet Scott is in no hurry to make this magnificent spectacle happen, with two and a half hours crawling by before they meet. Michael Mann did the same with De Niro and Pacino in Heat, but in that case each character's story was so strong it was a treat to wait. In American Gangster, it's a chore.

Washington plays Frank Lucas, thug underling turned dapper mob boss. When we first meet him, he has tied a man to a chair, set fire to him, and pumped bullets into the body to make sure he gets the message it's not a good idea to cross old Frank. In the way of every screen gangster from Cagney on, Frank is half-charmer, half-psychopath: one minute the twinkly-eyed rogue, the next a crazed animal whose violence seems to erupt out of nowhere. Washington, so often the stand-up guy in his movies, is clearly revelling in the chance to get his Training Day game on again. Judging by the excessive amount of screen time he devotes to Washington, Scott can't get enough of it either.

Another time, another place, Frank would be the CEO of a major corporation. As it is, he takes the patch he has (Harlem) a commodity guaranteed to sell (drugs) and sets about creating a monopoly. His way of getting the product into the country is as ingenious as it is appalling, and he makes a lot of money fast. While his life is on the up, Detective Richie Roberts (Crowe) is being divorced by his wife and frozen out by colleagues. Roberts is the cliche cop - womaniser, gambler, shouter. He is distinguishable from his fellow officers only in his unwillingness to take a bribe. As in Serpico, Scott shows how backhanders became a way of life for police, and others, in seventies America. Briefing his new anti-drugs team on the task before them, Roberts says: "If you stop bringing dope into this country, about 100,000 people are going to be out of a job."

Roberts and Lucas are a fascinating pair because they are so much alike. Each is a driven outsider operating to an unorthodox moral code: Lucas dishonest in how he earns a living, Roberts in the way he treats those closest to him. Instead of exploring this further, Scott opts to wander off on a self-indulgent trip through the period. He's by Lucas's side in his odyssey through nightclubs, parties and prize fights, his cameras lingering on the cars and the clothes, taking in every gaudy detail. With the predictability of night following day, he overlays one extended sequence with Bobby Womack's ghetto-fabulous Across 110th Street. This meander down seventies lane is a gas at first, but after a while the nostalgia grows old. Crowe, meanwhile, like the rest of the fine cast, barely gets a look-in. Nice centre parting though, Russ.

As Lucas and Roberts edge closer to each other, the pace quickens. The decade of excess is coming to an end; sleazy old New York and New Jersey are being cleaned up to make way for the financiers and tourists. Scott wraps up the film in a tearing rush, as if the fate of his main protagonist was of little consequence. In the few moments they do share in front of the camera, Crowe and Washington make an intriguing combination, Crowe's lumbering roughness contrasting with Denzel's legendary silkiness, but there's no chemistry, no fireworks, no T Rex and King Kong moment.

Strange that Scott should have given so little screen time to Crowe given how often the pair now work together. Their next movie is Body of Lies, in which Crowe plays a CIA boss to Leonardo DiCaprio's agent in a war on terror thriller. Let's hope the hairdresser count stays low.