Connoisseurs of singing rarely hold moderate opinions, and there is hardly a singer who elicits more adulation and disapproval at the same time than mezzo-soprano Cecilia Bartoli, who opens Oct. 16 in a new, made-to-order production of Rossini's La Cenerentola at the Metropolitan Opera.

Bartoli is more than a singer: She is a marvel of marketing. She sells out any house she appears in and moves enough CDs to qualify as a pop star--1.3 million since her first recording appeared in 1988. Her recordings outsell those of every other classical artist except Luciano Pavarotti. At 31, she has already had more ad hoc laurels lavished on her than many legends receive even posthumously. A chef in a San Francisco restaurant has created "Ravioli Bartoli." One of the Chunnel trains that link England and France is called the "Cecilia Bartoli." There is a spate of worshipful Web sites devoted to her, and a breathless and wildly premature biography by Kim Chernin and Renate Stendhal, Cecilia Bartoli: The Passion of Song, which describes her as "a sensuous, embodied angel, standing quietly on a concert stage as she report[s] back to God about the mysterious joys and sorrows of human existence." Another book about Bartoli and her contemporaries is being written by Manuela Hoelterhoff, the former opera critic of the Wall Street Journal, who is now a member of its editorial board.

D oes Bartoli live up to the hype? She is certainly a fine technician who never sacrifices passion to precision. Listen to her roulades--virtuosic runs of notes on a single syllable--and you'll hear each note as a distinct link in a long, twirling chain and each word being accorded its proper weight. Her new CD, An Italian Songbook, for instance, opens with one of dozens of settings Rossini made of two short verses by the 18^th-century poet Pietro Metastasio, "Mi lagnerò tacendo" ("I will complain silently"). It is a jovial anthology of trills, runs, grace notes, mordents, and melismas, with a section of operatic recrimination. Bartoli finesses the tension between glum words and sprightly notes right up to the end, when the music's exuberance finally overwhelms self-pity.

Bartoli's critics have argued that her voice is too petite to reach the outer spaces of the cathedral-like Met and that her repertoire is too narrow, fusty, and obscure. She has sung no more than a handful of operatic roles onstage, mostly Mozart and Rossini, and she has made a specialty of the songs that kept Rossini busy after his opera-composing career had ended. Bartoli's style of fussing over every syllable has been called twitchy, belabored, and self-consciously adorable. Critics also draw ominous comparisons to Kathleen Battle, the soprano whose gifts have been eroded in recent years by an uncontrollable tendency to simper. Like Battle, Bartoli has allowed her stage manner to become downright weird. On the video of her Houston Cenerentola, from 1995, her eyes pop out, her dimples deepen, her chin recedes unflatteringly against her neck, her head springs forward and back from her shoulders as if she were doing the Funky Chicken. Bartoli does not have a diva's sense of dignity.

After years of disarming girl-next-door normalcy, however, Bartoli may have begun indulging in the clichés of diva naughtiness, such as a habit of canceling appearances, including a much-publicized Battle-like spat with the Metropolitan Opera's general manager, Joseph Volpe. Bartoli has consistently said she is just protecting her health. Hoelterhoff recently told the New York Times that at least some of the cancellations had to do with the condition of Bartoli's brother Gabriele, who has had surgery for a brain tumor.

There is a certain irony to the accusation that Bartoli's voice and repertoire lack substance. Bartoli has said that she finds the wry humor of the 18^th century more modern than the grandiloquence of Verdi and Puccini. Her voice matches her sensibility: It is better suited to the human-scaled works she sings than to the colossal operas that today's colossal houses were built for. Opera houses have long been designed to be one-size-fits-all, but Rossini's domestic comedies inevitably seem dwarfed by a stage ample enough to contain the struggles in Wagner's Valhalla. It is not Bartoli's voice but the operas themselves that are too small for houses like the Met--or rather, the Met is too big for all but the grandest of grand opera. Under James Levine, the Met has barely dabbled in bel canto, the finespun singer vehicles of the early 19^th century, favoring instead the titans of opera. If La Cenerentola--Rossini's version of the Cinderella story--has made it to the Met for the first time in the company's history, it is only because Bartoli brought it there.

F or now, people seem willing to allow Bartoli to perform repertoire that would otherwise seem destined for the remainder bin. She has championed the music of Pauline Viardot-Garcia, herself a Rossini mezzo-soprano in the 19^th century, and led a campaign to rehabilitate Haydn's almost-mythological cantata Arianna a Naxos and his virtually unknown opera Orfeo ed Euridice. One reason she can get away with this is that she is a dazzlingly expressive singer, guiding the listener through the emotional landscape of a piece with every small push of her voice. She colors the first recitative in her recording of Haydn's Orfeo ed Euridice with more brilliant hues of grief than I've ever heard before."Sventurata!" she cries ("Unhappy me!") in a voice full of pain. "A thousand dismal thoughts"--her tone darkens--"constantly cloud my mind," and the notes acquire a ravishing mistiness that is partly unsung air.

To my mind, Bartoli's breathiness is central to her appeal. She can command a full, chesty holler when she needs to, but in quiet passages she tends to produce a gentle haze of breath around each focused note. This hiss, the noise level in her voice, like Glenn Gould's humming as he played the piano or the squeaking of Andrés Segovia's fingers on the strings of his guitar, makes her unusual today, when most singers are trained in the discipline of steely perfection and when our ears have become accustomed to the sharp-edged sounds of a compact disc, surrounded by undiluted digital silence. It is as if, by this deliberate smudging, Bartoli were giving old songs an antique timbre.

Bartoli's effort to cast herself as an anti-diva, at least in her stage performances, has worked. Some critics were scandalized, but more were charmed, when she made her Met debut two seasons ago as Despina, the servant girl and fifth wheel to four malicious and befuddled lovers in Mozart's Così fan tutte. She did not enter down a curving staircase, but rather toting a house on a rope like a barge--and made it immediately clear that she was carrying the whole production.

Bartoli is a wildly popular singer of relatively unpopular music, a singer who could probably fill stadiums, but prefers to keep her audiences at close quarters. Last year, she opted to give a recital in Lincoln Center's Alice Tully Hall, rather than in the larger Avery Fisher Hall (to the chagrin of the presenter). She has been leaning on the Met to find a more intimate venue for her projects. She has gravitated toward the stripped-down aesthetic of original-instrument ensembles. While Pavarotti and his two sidekick tenors have been turning opera into a mass-appeal juggernaut, Bartoli has been trying to shrink a bloated genre. That's the effort for which she really deserves megastardom.