Simon Price on pop: Would the real Biffy Clyro please stand up and rock out?

Sincerity – unthinkingly accepted as a positive attribute in a rock band – is a false friend. Biffy Clyro "mean" it, there's no doubt. But what "it" do they mean?

Scour Biffy's lyrics and you'll struggle to divine what they're trying to say. One early single, which berated an unspecified "you" who had "all these great answers to all these great questions", made me side with the addressee for at least having a position. Tonight, the only thing about which the bare-chested Simon Neil is vehement is his opposition to sitting (a gruff "if you're not standing, get up off your arse" is oft repeated).

Of course, if the stupidly named Kilmarnock trio truly rocked, they wouldn't need to ask. Occasionally, they do. "A Day of ..." has passages of Motorhead-esque thrash; "There's No Such Thing as a Jaggy Snake" a tough, Thin Lizzy feel. Most of the set, however, consists of airbrushed bluster like "Victory Over the Sun", unanimously declared "the piss song" by the sudden throng at the urinals.

I'm not immune to the appeal of Scot-rock: I was a teenage Big Country fan. And Biffy Clyro share more than a little of their DNA with that generation of arena-rockers: the one-note bassline in "Bubbles" is straight out of Simple Minds' "Waterfront". They play to a similar demographic: lads with "tribal" tattoos, and the lasses who go out with them.

Biffy's success is the result of a Venn-diagram approach. They're not a metal band, nor an emo band, nor an indie band, nor a pop group, but exist at the intersection of all four, pulling in enough fans of each to fill arenas, and to afford a giant stage set with staircases, flamethrowers, confetti cannons and filigree red lasers lacing the air like a spider's web.

It's a lucrative formula, all right. Biffy Clyro are as close to the centre of the mainstream as it's possible to get. And tonight the mainstream dutifully rises to its feet as if in the presence of royalty. God save Biffy Clyro. They mean it, maaan.

Mercury Rev (Koko, London ****) have rung the changes. They began as psych-rock crazies with songs that grew from nothing to everything-including-the-kitchen-sink in six minutes, during which singer David Baker, who looked like a Cuckoo's Nest escapee, would be joined by two dozen children in bee costumes. After Baker left, they achieved their greatest success with the grandiose Deserter's Songs and All is Dream, employing symphonic strings, a musical saw, and Jonathan Donahue's Neil Young impression.

They're into their third phase now. I'm not sure even they know what it is, but it's affording them the freedom to perform a live score to the charming 1956 film classic Le Ballon Rouge. Donahue & co choose to do this with one ominous sustained chord, overlaid with layers of vibraphone and glockenspiel. When the antics of the red balloon and the Parisian schoolboy get hectic, a pounding beat kicks in.

Why are they doing this? What will they do next? I, for one, hope their motives and movements remain as capricious as those of that balloon.

Critic's Choice

Dexys prepare for their nine-night residency at the Duke of York's Theatre, London, with warm-ups at The Apex, Bury St Edmunds (Tue); Komedia, Bath (Wed); and Guildhall, Gloucester (Sat). Meanwhile, British Sea Power take their new album Machineries of Joy to Oran Mor, Glasgow (tonight); Metropolitan University, Leeds (Tue); Rescue Rooms, Nottingham (Wed); and Gorilla, Manchester (Thu, Fri).