In terms of commercial success, Rodrigo y Gabriela just get better and better. It would have once seemed inconceivable that a duo playing instrumental pieces on acoustic guitars would be able to pack out Koko - and this gig is just a curtain-raiser for a major autumn tour. But when it comes to music, things are not so clear cut. Ever since they made that extraordinary leap from busking on the streets of Dublin to touring the world's concert halls, the two young Mexicans have dazzled audiences with their blend of rapid-fire, perfectly synchronised strumming and melody lines, with influences that range from Latin and jazz to the heavy metal riffs they started out playing in rock bands in Mexico City.

They have a unique style, but are faced with the problem of how to develop. Their new album (reviewed in today's Film & Music section) succeeds by adding new textures and instrumentation, yet this live show highlighted not only their brilliance but also their limitations.

It started, in impressive form, with a set of pieces that included the free-flowing Hanuman and the Hendrix-inspired Buster Voodoo, both from the new album, proving that the interplay between the duo is as intuitive and immaculate as ever. They provided greater range to their sound with the use of guitar pedals and effects, plus the brief introduction of the Latin cajon box drum.

But it wasn't quite enough. The first hour was absorbing; then they began to sound repetitive, despite solo spots and a classy reworking of Stairway to Heaven in the encore. There was an overemphasis on furious riffing, accompanied by Rodrigo's new line in rock hero poses - he often played while kneeling. It's what many in the crowd clearly wanted, and some left during the slower passages. But these two great guitarists are surely capable of more.

At Hammersmith Apollo, London, on 23 November. Box office: 0844 844 4748. Then touring.