Rowan has it. Denis has it. But Harry has most

It's one of advertising's oldest ploys, but celebrity endorsement is an increasingly risky proposition. Pepsi must rue the day it signed Michael Jackson and Madonna. And BT must be wondering why Bob Hoskins uses Mercury for long-distance calls. So why do agencies stick with it? "It brings immediate recognition for your brand," explains Grant Duncan, managing director of GGT Advertising. "If it works faster, ultimately it will be cheaper."

It is easy to lose sight of the benefits when you realise what some "personalities" charge. The going rate is said to be around pounds 50,000, with a sliding scale depending on star status. Jane Horrocks was rumoured to be getting pounds 6,000 a day for her appearance in a recent Tesco ad. Michael Barrymore got pounds 250,000 for plugging Cadbury chocolate fingers and Dame Edna twice that for flogging the same product.

Too expensive? Then why not create your own personality? Levi's did it in the Eighties with Nick Kamen and his launderette strip; sales of 501s shot up by 800 per cent. And Safeway is in the money thanks to Harry, the child with the oddly adult voice-over. The cult of the personality brings results. Here's the proof ...

When Abbott Mead Vickers BBDO launched its "recipe" for Sainsbury's in May 1991, it proved to be a shelf-stacker's nightmare. The first commercial featured Selina Scott and called for mozzarella cheese. Sales rocketed by 1,000 per cent when the ad aired. Catherine Zeta Jones's penchant for creme fraiche had a similar effect. But the late Robert Morley's tasty filo parcels took the biscuit; the ad had to be yanked because supply simply couldn't satisfy demand.

Fronted - literally - by the supermodel Eva Herzigova, TBWA's press and poster campaign for the Playtex Wonderbra made headlines on Valentine's Day 1994. Suggestive one-liners("Hello Boys") and Ellen von Unwerth's saucy photography proved a potent combination. Playtex reports a weekly increase of 41 per cent year on year. Some stores are shifting 200 per cent more of the booster bras, with more than 1.5 million bought in the UK over the past 12 months. The sassy campaign is soon to go international.

Rainey Kelly Campbell Roalfe's campaign for Virgin Atlantic Airways' Upper Class was the first time that Terence Stamp had appeared in TV advertising. In a relaxed manner, the six commercials put over highlighted product benefits such as leg-room, a chauffeur service and an in-flight bar area. Clearly, Stamp has what it takes: first-time bookings for Upper Class rose by 31 per cent year on year, all the more impressive considering that BA outspends Virgin something like five to one.

Since BMP DDB Needham's "No nonsense" campaign for John Smith's Bitter broke three years ago, sales have increased by 65 per cent; it is now the brand leader in the UK bitter market. The comedian Jack Dee, who co-wrote the ads, is the laconic spokesman, clearing the set of singing penguins, all the better to retain his "hard man of comedy" image. In March, Dee launched John Smith's Extra Smooth: it sold more in its first six months than any other new beer. Because of a conflict of interests (BMP won the massive Budweiser account), the account moved to GGT Advertising earlier this year. It has renewed Dee's contract for a reported pounds 200,000.

Safeway has announced record half-year profits of pounds 214m and will be creating some 7,500 jobs. These figures, it believes, are almost entirely down to its pounds 10m campaign masterminded by Bates Dorland. The nation has gone potty for Harry (played by three-year-old Jack Hanford), who comes complete with a Look Who's Talking-style voice-over from the actor Martin Clunes. When Harry isn't dumping groceries, you'll usually find him pelting his parents with Safeway's finest. The little angel.

Gary Lineker's unlikely performance in BMP DDB Needham's campaign for Walker's Crisps has increased sales by 29 per cent since its January debut. The "No More Mr Nice Guy" ads see the clean-cut footballer stealing crisps from young boys. The first ad generated numerous complaints to the ITC, but Walker's certainly isn't whingeing; it is selling nearly four times as many crisps as its nearest rival. A new commercial, now in production, teams our Gary with that "misunderstood genius" Paul Gascoigne.

Holsten Pils has stuck to personality-based campaigns, though not the personalities themselves, moving from Donald Pleasance to Griff Rhys-Jones, to Jeff Goldblum, to the American comic Denis Leary. "They've all reflected the brand's needs at a particular time," explains GGT's managing director, Grant Duncan. "Leary restates uncompromising product values." In the past 12 months sales have gone up by 14 per cent in multiples and 25 per cent in independent grocers and off-licences; one in every four bottles of premium packaged lager sold in pubs, clubs and bars is a Holsten Pils.

The antics of the MI6 agent Rowan Atkinson and his Barclaycard-toting sidekick, Bough, first aired in 1991. BMP has completed 14 ads, all of which see Latham landing himself in it in exotic locations. Applications for Barclaycard are up by 40 per cent year on year; Barclaycard spending is up by 12 per cent year on year.

Heeding the advice of Abbott Mead Vickers BBDO, Ikea used not one but 20 hyperactive Keith Chegwins to remind customers that it gives value for money. A follow-up ad featured 271/2 Nicholas Parsons. In the Leeds area, where the Cheggers ad was used to launch a new store, an astonishing 23 per cent of the local population checked it out in the first month, and 75 per cent of those aware of the ads said they were likely to visit. So there.