Jason Braatz has been in the city's floral business for six years — enough time to know that it pays to keep Yelpers happy. And Panthea J. is not happy. "Sadly, it was not the dramatic floral marvel that I had envisioned," she sighed about a bouquet from Braatz' Rossi & Rovetti Flowers in her review at Yelp.com, "but a small and slightly wilted version that wouldn't have compensated for cutting [the recipient] off in traffic." She gave the florist just two stars out of five. Ouch.

Panthea J. is one of thousands of ordinary people who've rated eateries and theaters, dentists and dive bars, on the popular consumer Web site that relies on user-generated reviews. While a couple of years back no one would have given a hoot what Panthea J. thought about her bouquet, now hundreds type in "florist" on Yelp each day to read the recommendations of consumers like her. Though he has a solid 4.5-star rating overall, Braatz claims a positive review at the top of his page brings in 100 orders. He suspects a negative one scares off about the same.

Knowing this, Braatz decided he had to make nice with Panthea J. He dashed off several e-mail equivalents of thanking Her Highness for the public flogging: "I appreciate your sincerity and the time you had spent in writing it," he wrote. "Thank you so much once again! If there's anything else we can do, consider it done!"

It wasn't the only time Braatz had tried to buy Yelpers' goodwill. His early offers of a free bouquet and $100 for removing their reviews smelled funny to some Yelpers ("Money or flowers cannot at this time make up for not receiving my Valentines bouquet," one sniped in an e-mail response). So Braatz launched a customer service shock-and-awe campaign, hoping to goad the grumblers into updating their reviews to five stars without explicitly asking. He filled their requests one by one: Madagascar jasmine delivered the same day? No problem. The 20-centimeter stem roses for the same price as the ones half their length? Done. The clients later deleted or toned down their bad reviews.

Panthea J., however, turned out to be a more difficult case.

Months after Braatz e-mailed her promising to do anything to make her happy, a message from her popped into his inbox: She said she wanted to take advantage of Braatz' earlier offer. "Yes, if you send me a free arrangement, I would be happy to provide feedback and, though unsolicited, would be happy to revise my review accordingly," she wrote. Braatz sent three free arrangements, hoping she would follow through.

But the two-star review stayed put. Braatz kept trying to please her. Panthea had more than 200 friends on the site and was a member of its Elite Squad — clearly a woman of influence in the Yelp kingdom. If he kept sending bouquets, she might update her review. On the other hand, if she was displeased, she could write something far worse.

Delivery receipts show that Braatz sent at least four more bouquets to Panthea's address, unaware that she had since moved and apparently never got any of them. He claims they were all sent in response to further e-mailed requests; she says he's fabricating the e-mails to discredit her bad review. While she acknowledges she "dropped the ball" on updating her write-up of Rossi & Rovetti, she insists she never requested a thing after the initial bouquet.

Whatever the case, $1,000 worth of flowers later, Braatz was unable to get a more favorable review from Panthea. He figures it's just the cost of doing business these days. Overall, he reckons, Yelp has been a boon to the shop tucked in the Bank of America building on California Street. He credits Yelp with helping quintuple his annual gross income to more than $1 million since he bought the company in 2003. He says Tom Hanks' personal assistant found him via Yelp and had him fill a Porsche with more than 1,200 roses as a gift to a niece.

"It's become one of our most powerful advertising tools," Braatz says. But that doesn't make having to placate Yelpers less annoying: "The power for them to put a bad remark on can crush us. So we have to do 10 steps above everything. ... We literally overdo customer service."

Interestingly, Rossi & Rovetti symbolizes the city's old power elite. Braatz has learned that the business, founded by the family of Angelo Joseph Rossi, who went on to be strike-busting San Francisco mayor from 1931 to 1944, likely had ties to organized crime and delivered its roses in alcohol during Prohibition. If your flowers arrived wilted back then, you probably kept it to yourself.

But at the turn of the 21st century, the Internet has made it possible for members of a new power class to say how business is run in town. They call themselves Yelpers, and they can have a major impact on a store or restaurant's bottom line. Businesses' only choice is to please them — or not.

The worldwide headquarters of Yelp spreads out on the third, seventh, and eighth floors above Rochester Big & Tall at the corner of Mission and Third streets. Inside, amid shuffleboard and Ping-Pong tables and a pen for the CEO's dozing dog, young tech nerds with headphones and zip-up hipster jackets — not a tie in the place — silently type code and manage complaints for a site that attracted just shy of 20 million unique visitors last month. For reference, that's not even a third of those of Web 2.0 giant YouTube, yet Yelp's steady uptick in visitors is likely making the older review site, Southern California–based CitySearch — whose visitors plateaued in the 30 million range last year — a tad nervous.

Jeremy Stoppelman, Yelp's photogenic 31-year-old chief executive, founded the site in 2004 with fellow University of Illinois engineering school alum Russel Simmons, after the two met while working at PayPal. Stoppelman was inspired to start Yelp while looking for a doctor. With venture capital from their PayPal boss, the two launched a site where users could ask friends for referrals to businesses, but discovered that users preferred to write unsolicited reviews — for free.