Tasty, certainly, have been its sales figures. Audi, owned by the German car giant Volkswagen, sold 568 of them in the UK in 1993. Last year, sales rocketed to 989, adding pounds 10.2m to Audi UK's annual revenues. Sales of the 2.6E this year will be even higher.
What catapulted demand in this way? Was it the powerful 150 brake horsepower V6 engine? A top speed of 130mph? The luxury interior with leather-trimmed sports seats?
No doubt all these contributed. What really seems to have made the difference is that last February the Princess of Wales started driving one.
The ensuing publicity for the car has been breathtaking. The paparazzi know its sleek lines almost as well as the Princess's own face. In a thousand TV reports, she is seen getting into it, getting out of it, and sweeping imperiously out of the gates of her Chelsea health club.
It is the kind of free publicity the average marketing director would die for. Re- peatedly appearing on national television is a coup in itself. Having the endorsement of the world's most glamorous woman is something else.
Audi is obviously delighted with the publicity. According to Natalie Dinsdale, Audi's corporate communications manager: "She obviously has increased the profile of the car. How much of the sales increase is attributable to her we don't actually know."
The Princess herself is rather more blunt about her unparallelled ability to shift merchandise of all sorts. As she told Panorama last week, "You see yourself as a good product that sits on a shelf and sells well, and people make a lot of money out of you."
She was, of course, talking primarily about her ability to help sell newspapers and magazines - the comment was in response to a question about media scrutiny. And newspapers last week enjoyed bumper sales precisely because of the sensational news value of her interview.
Gary Lott, circulation manager of the Daily Mirror, estimates that tabloid newspapers added a total of half-a-million copies to their sales last week because of the Diana story. Precise figures will not be known until this Wedesday, when all the "returns" - unsold copies - are counted.
On Tuesday alone, the day after Panorama, the Mirror added an estimated 180,000 copies to its normal 2.4 million circulation. About half of that was probably because of new readers picked up as a result of the closure of News International's Today. But that still leaves Diana boosting Mirror sales by almost 100,000 copies.
It is not just newspaper publishers that benefit. In the case of the Mirror, corner shops and newsagents get 7.15p for every extra copy sold. Such wholesalers as WH Smith and John Menzies get their cut. Mirror Group Newspapers (co-owner of the Independent on Sunday) receives about 15p a copy of the 28p cover price.
Perhaps surprisingly, the broadsheets benefit as much as if not more than the tabloids from a good royal story. Sales of the Independent on Tuesday were up by between 5 and 7 per cent, according to initial estimates. Not only does added circulation boost direct sales revenues, if sustained, it also raises the rates newspapers may charge advertisers.
Royal events undoubtedly stimulate newspaper sales, says Derek Terrington, media analyst with stockbrokers Kleinwort Benson: "The tabloids have fed off the Royal Family for years and will continue to do so. The Princess of Wales is one hell of a story, and not just in this country. You can see it as equivalent to a special promotion or bingo. In that sense she boosts sales on the day."
But newspapers are lucky to add more than 5 per cent to their daily sales from royal news that everyone has. It still takes a genuine scoop to produce a big sales increase. Last Tuesday's sales performance by the Mirror was dwarfed by its Fergie foot-kissing exclusive, when it put on an extra 1.7 million. Nor is Diana, qua Diana, sufficient.
Sally Cartwright, publisher of Hello! magazine, part of the Spanish group that publishes Hola!, says: "Given the right moment, the Princess of Wales is very good for magazine sales. But you can't just stick her on the cover and expect an enormous sales increase. It depends on whether she's been in the news lately. You can't use her willy-nilly. She isn't a magic wand." She recalls a time when the Princess was used on three consecutive Hello! covers. "It was very much the law of diminishing returns. By the third issue there was very little uplift."
Magazine publishers pay some enormous sums to charity in return for the Princess agreeing to a photo-shoot. But the numbers still stack up. Not only do they recoup the payments from additional sales and advertising revenue; they also sell rights to the photographs worldwide.
The American magazine Harper's Bazaar was last week reputedly hawking British rights to new pictures of the Princess for $5,000 (pounds 3,200) each. They duly appeared, first in the Daily Mail, then in the Sun.
Diana's selling power is not confined to the newsagent's shelf. Like Midas, she helps turn any product or service or store or restaurant she uses to gold. Harrods, Harvey Nichols, San Lorenzo, Evian water, Reebok trainers - all have basked in the glow of her patronage. British fashion designers for years enjoyed her loyal custom. In public, she would only wear British. That has changed: she is now seen in Versace and Chanel.
Richard Branson recently joined the ranks of businessmen favoured by her patronage. She started very publicly wearing a Virgin Atlantic sweatshirt. She met Mr Branson at the naming ceremony of Virgin's first Airbus A340 a couple of years ago.
"She requested the sweatshirt when flying with us on one of her recent trips, and we were delighted to oblige," says a Virgin spokeswoman. "We're delighted she chooses to fly with us. We're even more delighted she wears the sweatshirt."
The seal of approval from the Princess of Wales is like no other. She has no official royal warrant. These are confined to the Queen, the Queen Mother, the Duke of Edinburgh and the Prince of Wales. But while royal warrants mainly flatter the proprietors they are awarded to, Diana's unofficial endorsement delivers worldwide publicity.
Her impact is colossal, but easily explained. Nigel Evans, editor of Majesty magazine, says: "She's the most famous woman in the world. She's the most popular woman in the country. She's royal. She's very attractive. And she has a charisma lacking in many other members of the Royal Family.
"People are fascinated by her . . . by the products she uses, the places she goes and the clothes she wears."
And, he adds, she is made of Teflon. The media have only turned on her twice: once over alleged nuisance calls to Oliver Hoare, once over her alleged
role in the break-up of Will and Julia Carling's marriage. Both times she bounced back. "She's unassailable," says Mr Evans.
Peter York, the management and marketing guru and co-author of The Sloane Ranger Handbook, explains: "It's not snob appeal, it's film star appeal. It would be snob appeal if she were a regular aristo, but she's a highly irregular aristo."
Last year, Majesty calculated that the Princess generated pounds 14.5m-worth of free publicity for products she bought, by measuring the column-inches of mentions in tabloids.
The San Lorenzo restaurant, a favourite lunching-spot of the Princess, enjoyed pounds 25,000 of free advertising, the magazine calculated. Harrods got pounds 21,000, the Chelsea Harbour Club pounds 73,000, Chanel pounds 12,000 and Evian water pounds 59,000. Disney World, where she took sons William and Harry, garnered more than pounds 129,000-worth, Marks and Spencer pounds 87,000. Her favourite charities benefited too, the Red Cross getting pounds 40,000-worth and Relate pounds 44,000.
Dominic Mills, editor of Campaign, the advertising trade's magazine, points out her image is in tune with the aspirations of a huge chunk of the population. "She's a role model for all women who consider themselves in her age category, which means anyone from 20 to about 50. And she's a role model for women who have gained their independence, or aspire to more independence, or who want to assert themselves in their relationships."
A big theme in recent advertising has been the targeting of independent women. Ads for Ford Fiestas, Peugeots and Metros have all stressed this aspect. The Volkswagen "Just Divorced" ad was a striking example.
Could we one day see the Princess formally endorsing products in the way that Lauren Bacall endorses J Sainsbury, or Anita Roddick punts American Express? Unlikely, says Mr Mills, but if she was prepared to, the rewards could be spectacular. Michael Jackson is reputed to have received $15m from his tie-up with Pepsi, before it was dropped in the wake of the child sexual abuse allegations "She'd be up in that league," says Mr Mills, who reckons her Panorama performance makes her even more marketable. "It was brilliant. The impression of total honesty was terrific."
Diana as product sponsor may be a bit hard to swallow. But Diana as ambassador for British exporters is wholly feasible. She can open doors anywhere in the world. And British trade missions would undoubtedly receive more attention with her as the figurehead than those led by Michael Heseltine or the Prince of Wales.