Pierre Boulez Weekend, Southbank Centre

William Glock once claimed that Pierre Boulez could literally vomit at music he believed to be substandard. I wonder what he would have made of my friend, who fled at the interval of the opening concert of the Southbank festival on Friday blaming Boulez's Domaines for setting off a panic attack.

Her physical response was certainly a welcome corrective to the nonchalance with which the critical world increasingly greets Boulez's language, many of whom still insist that the days of serialism provoking anger or revulsion are in the distant past. Boulez can still upset. He even upset me a few times.

Mostly, however, we all marvelled. One can't stay upset for long with a composer whose gestures - the arpeggiated tumbling, the trills, the Indonesian shimmying, the Lisztian razzle-dazzle - are so fundamentally attractive. One can't stay upset with a composer, who, though he may have started off as the great Moses-like law-giver, also became the era's chief punkish law-breaker. One of the major focuses of the weekend was on Boulez's transgressions - his obsession with trying to loosen up his language. One idea he wrestled with for decades, with mixed success, was open form: the idea that, if one allowed performers to choose certain aspects of a piece, the work would become free.

Barbara Hannigan's ecstatic rendering of Pli selon Pli ranged from Monroe-like whispering to shrill cries

We witnessed the progression of this idea from its Cagean beginnings in the Third Piano Sonata (1955-57) - an infuriatingly stop-start piece even in Pierre-Laurent Aimard's intense reading - to its theatrical maturity in Structures II (1956-1961). But on the opening night, the Royal Academy of Music Manson Ensemble focused on two applications of chance that resulted in something quite suburban. In Domaines for clarinet and orchestra (1968) choices are made prior to the start on which pages to use, which passages to play. But with homework done, nothing more modern than a picaresque unfolds. The caddish clarinet of Elaine Ruby pays two polite visits to each of six charmingly well-behaved chamber groups, who offer an opening brass fanfare, a pastoral eddy and a springing to life in several genteel dances. In all but name, we were in the winning company of a courtly Baroque suite.

In Rituel in memoriam Bruno Maderna (1974-75) the aleatoric intrusions do even less. Which is a shame because no work needed a Boulezian lightning strike, a fast-bowled arpeggio on marimba, more than this uncharacteristically four-square piece, whose barrage of hammered-home sounds undermines any of the title's hopes of investigating memory. It also suffers from a stylistic closeness to Messiaen's Et expecto (1964), which Rituel struggles to out-anger. Only in the second book of Structures does open form really find its feet. In it, two pianists play, chase and tussle over 40 minutes, the second piano goading the first into monstrous aleatoric outbursts. It culminated in a final epic low cadenza, a rare glimpse of hell.

Before Cage had introduced him to chance, Boulez relied on good old-fashioned Romantic storms to crash through the well-ordered serialist houses that he had carefully constructed. In Notations six and nine register is exploded to create canvases of chaos. In the First Piano Sonata an emotional and physical savagery continually scatters the ideas outwards. It was a shame more people weren't there on Sunday morning to witness Aimard's ferocious performances. 

The Second Piano Sonata is different again in its aims. Unlike so many of Boulez's works it doesn't take entropy as its starting point. Rather, we are back briefly (oddly) with Beethovenian brick-laying - though with monstrous, sky-scraping consequences. Tamara Stefanovich for some reason got no page-turner and so had to juggle incredible pianistic dexterity in the Himalayan final movement fugue with equally incredible page-flipping.

By the 1990s Boulez had moved on from open form and was creating spontaneity through electronic intrusion. This is where Boulez comes slightly unstuck; after all, nothing dates quicker than technology. The clichés of fashion therefore linger in these works more than in any others. Anthèmes 2 saw the violin of Clio Gould draped in naff, gypsy wedding-like bridal gowns of reverb sound, the slimy trains making an ondes Martenot of the instrument. Meanwhile, the looping polyphony came across as little more than an academic Minimalism. "... explosante-fixe..." (1991-93), in the hands of Peter Eötvös and the London Sinfonietta, didn't help the electronic case much. Miked from behind, the flutes were all breath and no sounding blow, which made for fuzzy listening.

One of the easiest ways to free strict musical forms - as Mahler and Wagner knew well - is to introduce language. In Pli selon Pli the rich visual imagery of Mallarmé's five poems gives Boulez the chance to free himself completely. A milky way of orchestral colour becomes the setting. We call mainly on those instruments that have little ability to sustain their sound - marimbas, guitars, harps. The result is a careful work, moving en pointe, word matching timbre matching word matching timbre. The ravishing but tidy ornamental garden that was summoned up by Boulez and the Ensemble InterContemporain might have become a little too ordered were it not for Barbara Hannigan's ecstatic rendering of the words, ranging from Monroe-like whispering to shrill cries.

All in all, a revelatory weekend. The chronological survey of the piano work on Sunday with Aimard's ever-helpful pre-concert analyses, capped off by Pli selon Pli, was the highlight. But as interesting as rediscovering the specifics of Boulez's genius was spotting the weaknesses. With Boulez, slow tempi are definitely an Achilles' heel. Time and again through the weekend a slackening of pace saw him come a cropper. He doesn't seem to know what to do with himself when at rest. Only in Pli selon Pli, with textural possibilities investigated in full, does Boulez seem to find a purpose for the moments of reflection. And it's a very French solution: beauty and description.