NEW YORK (CNN/Money) - At the height of his popularity, Michael Jordan wasn't just the best basketball player on the planet -- he was the most powerful marketing force, too.

Almost single-handedly, Jordan lifted the fortunes of several of the companies he represented. In 1998, Fortune magazine estimated that, through his on-court and off-court activities, Jordan had contributed $10 billion to the U.S. economy since his 1984 NBA debut. And the magazine guessed that Nike was the beneficiary of more than half -- $5.2 billion to be exact -- of this "Jordan Effect."

Jordan made it all seem so easy: Hire a cool celebrity endorser, pay him a few million (Nike reportedly paid Jordan somewhere in the neighborhood of $25 million a year at the peak of their relationship) and reap billions in added sales.

But Michael Jordan the marketing machine, much like Michael Jordan the Chicago Bull, was occupying rarified air. Down on earth, celebrity pitchmen never pay off in such grand fashion. And rarely can the terms be measured so explicitly.

"That is very difficult to do," said Brian Till, a marketing professor at St. Louis University's John Cook School of Business. "What we are talking about is disentangling the effect of one aspect of a brand's marketing and discerning what effect that one aspect has on sales."

Jeff Kanter, an analyst who tracks beverage stocks for Prudential, put it this way, "It's impossible to quantify."

Despite the muddy returns, the pace of celebrity pitches is as furious as ever. Britney Spears, Shaquille O'Neal, Tiger Woods, and Jason Alexander are all more likely than not to make an appearance in a TV watcher's living room on any given night.

And consumers are paying attention. According a recent survey that gauged viewer recall of television ads, five of the top 10 most successful spots in 2002 feature celebrities.

"It is very clear that celebrities can get an ad noticed," said Cheryl Idell, president of the Intermedia Advertising Group (IAG), the firm behind the survey. "Whether or not that ad goes on to sell the product is another story."

The Pepsi Spears campaign, which ranks first in the IAG survey, is perhaps the most prominent of the current crop. In February 2001, the beverage giant made Britney its all-time highest paid spokesperson with a contract thought to fit somewhere in between Venus Williams' five-year $40 million deal with Reebok and Tiger Woods' five-year $100 million contract with Nike. In return, she has danced for Bob Dole, gone back through Pepsi time, and taken a blow from Austin Powers.

On the surface, the business results have been good. In the four full quarters since Spears signed on last February, year-over-year sales in Pepsi's Pepsi-Cola North America division have climbed an average of 15 percent a quarter. And that has helped spark a 14 percent gain in Pepsi (PEP: Research, Estimates) stock since the campaign began.

Similarly, Alexander, whose unlikely turn as the "Chicken Fanatic" also elicits high marks -- two of the spots rank in IAG's top 10 -- has presided over a productive 10 months at KFC. Same store-sales are up in each of the 10 four-week periods for which Alexander has been the spokesperson (at an average of 4.5 percent per period). Stock in the parent company, Yum! Brands (YUM: Research, Estimates), has soared 41 percent during that stretch.

"Our measures [for KFC] -- everything from sales to advertising impact -- have gone through the roof, and Jason Alexander was a big part of that," said Jeff Mordos, chief operating officer of BBDO New York, the agency behind the Pepsi and KFC campaigns.

So it's fair to say that Spears and Alexander have had some impact on their companies' respective good fortunes. But before we can drop them in Jordan's territory, it's important to note that the products they represent are just pieces in their respective corporate puzzles.

In addition to Pepsi-Cola, PepsiCo also counts Frito-Lay and Tropicana, Gatorade and Quaker among its businesses. To paraphrase Neil Young, Britney doesn't sing for Tropicana.

She doesn't sing for Mountain Dew Code Red, Sierra Mist, SoBe, or Aquafina either. And it is those new products, not Pepsi proper, that have driven much of the growth within the Pepsi-Cola unit during Britney's reign. One Britney product, Pepsi Twist, is doing well, but Britney came aboard that ad campaign months after it left the station.

And Alexander is just one soldier in KFC's $200 million "War on Fast Food." As successful as the KFC spots may be, analysts struggle to gauge how many people the "Chicken Fanatic" is lifting from the couch and into the nearest purveyor of the Colonel's Original Recipe.

Of course, there is the rare case when the celebrity is so inextricably tied to sales that the impact is easy to grasp. In 1999, in what was thought to be the largest sum ever paid to an athlete endorser, Salton Inc. (SFP: Research, Estimates), makers of the George Foreman Grill, paid the former boxer $137.5 million in cash and stock for the lifetime rights to his name and likeness. (The company had been paying Foreman a 60% commission on Grill sales prior to the deal.)

It was a large pill for the $500-million company to swallow, but Salton had already essentially renamed itself "Foreman" and sales were soaring. Sales continued to soar in 2000 before dipping with the economy in 2001. Still, the company will do close to $1 billion in business in 2002, thanks largely to the unwavering strength of the Foreman brand.

PepsiCo and Yum! are in a different league than Salton, and it takes considerably more to move the needle.

And Mordos says plugging in a celebrity pitchperson is often only the third step in the advertising process, something to consider after the agency and the client have plotted strategy and conceptualized a campaign. Even then, the fit has to be just right.

"A celebrity is not the answer to an advertising problem," Mordos said.

Just ask WorldCom (WCOM: Research, Estimates). Five years after inking a deal with Jordan, the company is struggling to stay afloat. Which goes to show that sometimes even the coolest man on the planet doesn't pay.