The Roots are a favourite outfit among hip-hop/soul musicians. The veteran Philadelphia crew have moonlighted as backing band for Eminem and Jay-Z, dabbled with Erykah Badu and Jill Scott, and been widely namechecked. However, after six albums, a commercial breakthrough remains tantalisingly out of reach - although their live pulling power can easily sell out Manchester Academy.

It's easy to see why they command such respect. The sextet (usually, although musicians wander on and off the stage) speak hip-hop like a language, but switch into other genres with astounding musicianship. Their apparel reflects their eclecticism: a 1970s funk afro, Run DMC-style gold medallion, and a Ramones T-shirt. However, their hard-to-pigeonhole sound and image must make them a marketing executive's nightmare.

Their gigs are very different affairs from their records, with the fluid collective dabbling in everything from long dub excursions (which are fantastic) to an unexpected, audience-sung blast of Shabba Ranks' Mr Loverman. Soul drifts gloriously in and out of the mix, although there comes a point where you worry about the audience showing such a fetishistic interest in guitar solos, and wonder if you've come to a Genesis gig by mistake.

The Roots are at their best on direct, simple statements such as the Sly Stone-sampling Everybody Is a Star. Alas, spurred on by the crowd, the gig gradually becomes more indulgent as bass solos, odd amalgamations and dopey keyboard twiddly bits further summon up the ghost of prog-rock past. The Roots' proficiency is their undoing: they could probably turn their hands to everything from death metal to a hip-hop space symphony with a teapot solo. Terrifyingly, it's only a matter of time before they do.

· At Brixton Academy, London SW9, tonight. Box office: 0870 7712000.