Though Beyoncé Knowles is still a premiership pop star, scoring number ones effortlessly with singles like Beautiful Liar, her polish is cracking. R&B's leading lady saw her acting debut in Dreamgirls eclipsed by co-star Jennifer Hudson, and her last album B'Day was given such a lukewarm welcome, her dad was forced to publicly defend it. Now she faces a lawsuit from singer Des'ree, who claims the ex-member of Destiny's Child covered her song Kissing You without permission.

Like the star herself, smothered in dry ice and swathed in a shimmering, silver two-piece, The Beyoncé Experience - as her tour is tagged - is all smoke and mirrors. Knowles immediately discards her long skirt for a bum-skimming number and unleashes her finest solo moment, Crazy in Love, tearing around the stage like Lady Macbeth in the throes of insanity.

"How many of you guys got my first album, Dangerously in Love?" she asks. Despite a deluge of cheers, Knowles adds that she wants "to do something new". But the messy mediocrity of Freakum Dress and Green Light leaves Knowles relying on her booty and big voice to detract from her dwindling originality. She shakes, struts and thrusts like Tina Turner in her prime, but her voice, mauled by terrible sound, judders from booming to painfully shrill, stripping Naughty Girl of its cheekiness.

She is on safer ground with a Destiny's Child medley, allowing the crowd to sing for her. But the effort Knowles puts into her show is breathtaking. She writhes and poses with a team of dancers, runs through numerous costume changes, engages with her all-female band and pours her heart into every song, emoting winningly in front of a wind machine. When she breaks down in tears during Flaws and All, it is only the speed of her recovery that makes it appear an act. But that's the essence of The Beyoncé Experience, where style always overshadows substance, and professionalism replaces profundity.

· At Cardiff International Arena tonight. Box office: 0870 380 0017. Then touring.