A Film Critic confesses: I watched Spiceworld the Movie (PG) in a daze of pleasure. Some had predicted that it would prove to be its stars' obituary - but in fact, it's an uproarious pantomime of ironies that dramatises celebrity in a delightfully topical and sophisticated way. I think it's as important to the 1990s as A Hard Day's Night was to the 1960s. But I have a terrible feeling that nobody will take it seriously.

The big screen is not as flattering as your average tabloid snapper, and this is the first hurdle over which the Spice Girls have to jump. Some fare better than others: beetroot-red Geri Spice (Geraldine Halliwell) looks like a glamorous sister of Freddie "Parrot Face" Davies; from certain angles, Baby Spice (Emma Bunton) appears to have a completely ovoid head. And can they act? Well, Sporty (Melanie Chisholm) is full of Scouse nous, and should immediately be signed up for the next Ken Loach film. Scary (Melanie Brown) negotiates the clunkinesses of the dialogue with confident ease. As for Posh, well, perhaps Victoria Adams has carefully chosen to interpret her character as a woman so chronically stupid she can barely get her lines out.

The film's main subject is the relationship between the Spice Girls and their consumers. The script gives regular expression to a painful awareness of the ephemeral nature of their fame; Stephen Fry appears as a judge consigning them to the dustbin of pop history (the dread name of Gary Barlow is uttered). And, in another vignette, the Girls imagine themselves in the thrall of age and motherhood - Geri barely visible under trenchers of Bet Lynch make-up, and Sporty with a pineapple clip and baggy tracksuit added, so she becomes a dead ringer for Waynetta Slob. Most perceptively, the recent backlash against the Girls in the tabloids is confronted head-on, as slobbering Antipodean newspaper mogul Kevin McMaxford (Barry Humphries) sends one of Beelzebub's very own paparazzi (Richard O'Brien) to engineer their downfall.

The movie portrays the Girls as keen analysts of their own brand image. "I'm always going to be seen as Baby Spice," opines the blonde pig-tailed one, lolling on a swing with her chops round a Chupa Chup. And for a moment, you're struck with the terrifying image of a grown-up woman thrashing in the straitjacket of an absurd stereotype. This is Emma Bunton, a normal adult, compelled by marketing men to drag up as a powder-pink Kinderwhore for the benefit of the under-10s and their schoolgirl-fixated fathers. Tragic, isn't it? And from within this teeny-tart act, she's voicing doubts about the limitations - and perhaps the propriety - of such a pose.

But to suggest that the Girls' image is a ludicrous publicity stunt is to imply that their music is similarly daft and otiose. And if you're a gang of pop icons, there's a limit to how far you can satirise the processes of your consumption. Spiceworld shows its five heroines scorning the fabrications of evil tabloideers and mocking the fair-weather admiration of well-heeled blaggers. But they have to insulate their biggest contingent of hangers-on - their little screaming fans - from any such criticism.

Consequently, a sequence that begins with the Girls deciding to swap clothes, hair and mannerisms - so pointing up the vacuum-formed artificiality of their stage personae - is forced to conclude with a swift reversion to type. And by staging scenes in which the domestic manners of the Spice Girls conform to their emblematic public identities, the film thoroughly naturalises their respective Babyness, Poshness, Scariness, Sportiness and, er, Gingerness.

As if to compensate for this, director Bob Spiers has filled the film with personalities whose aura of weird self-parody is as bright as that of its stars. Gary Glitter has ended up on the cutting-room floor in alleged disgrace, but the Girls have the pantomime presences of Barry Humphries, Meatloaf and Roger Moore, to make them appear like actual human beings. Moore, who pops up as the Girls' behind-the-scenes-Svengali, spends the film in a sub-Bondian control room, where he shakes Martinis, strokes a menagerie of kittens and piglets, and slurs a series of cryptic proverbs into a perspex Trimphone. "When the speeding melon hits the wall," he croons, "it's Christmas for the crows."

The film's structure is similarly complex, and loops about like an all- singing, all-dancing Mobius strip. We follow the Spice Girls' unlikely adventures as two dollar-hungry Hollywood types (George Wendt and Mark McKinney) pitch possible film scenarios for a money-spinning cinematic vehicle. (The Spice Girls' manager, a deliciously apoplectic Richard E Grant, listens with varying degrees of interest.) At the climax of the film, the pitch and the plot run parallel as the Girls race to the Albert Hall on their Spice Bus, and McKinney simultaneously improvises a synopsis of the same events. Finally, as the credits roll, we see the Girls and their fellow actors preparing to shoot a scene from McKinney's pitch: a film that is clearly Spiceworld the Movie.

Complicated, isn't it? All this Byzantine complexity marks Spiceworld out as a terrifically important film. It'll be a set text on the cultural- studies courses of the 21st century. It's one of the most intriguing documents of post-modernity that film has yet produced, fusing the recursively self-referential qualities of Fellini's 81/2 with the disposable poppiness of Help! And I say that without a hint of irony.

Christmas week's other new film is Yim Ho's Kitchen (15), a lyrical, loopy comedy of love and bereavement based on the cult novel by the Japanese writer Banana Yoshimoto. Aggie (Yasuko Tomita) is a traumatised orphan taken in by her only living relatives, Louie (Jordan Chan), a Nintendo kid with a Gary Numan fringe, and his transsexual mother (Law Kar-Ying). Its a household with three lava lamps, a forest of cut flowers, and a plethora of smart utensils - that remain criminally unused until Aggie's arrival. Yim Ho dwells on the shiny, translucent qualities of household objects until they seem like mystical artefacts: mirrors, knives, cigarette lighters and vodka bottles all undergo this transsubstantiation. Mixing magic realism with the kitschy verve of more standard Hong Kong cinema, Ho's film is beautifully photographed and energetically performed, and glows with a sly, tender wit that would warm the cockles of the sternest Scrooge.

Finally, the BFI has dusted off The Magnificent Ambersons (U), the Orson Welles masterpiece that fell victim to studio meddling, and a popular contender for best film of all time. Viciously re-cut by RKO against the director's wishes, Welles's dynastic drama is the mutilated cadaver of a classic, but enough flesh remains for you to marvel at his supernatural understanding of the cinematic image. It's like gazing at a series of haunted magic-lantern slides - an incomparable company of actors (Joseph Cotten, Agnes Moorehead, Dolores Costello, Tim Holt) move through scenes of luminous artifice with an uncanny grace, all the time overseen by Welles's showmanlike narrator. "My name is Orson Welles ... I wrote and directed the picture," he rumbles, in conclusion. True; but you only wish it was truer.

Cinema details: Going Out, page 8.