A Temporary-Looking royal crest above the proscenium assures nervous patrons that they've come to the right place. Even with pounds 1.5 million spent on new paint and fittings, you'd not call Labatt's Apollo a lovely theatre. "Big" is about the best you can say of it, and "hard to miss". Overhung by the Hammersmith flyover, hedged by a four-lane roundabout, it leers its neon message across the monoxide haze to those emerging from the tube. Labatt's, the owners, brew lager, which they sell for pounds 2.40 a pint at the bar. To date, the place has run on rock gigs and Riverdance. Now it's playing host to the Royal Ballet.

But those who scorned the company's bid to attract that valuable "new audience" while the Opera House is rebuilt must prepare to eat their words. The season's opening night was a triumph. Most of the 3,500 determined to inch and elbow their way in to Romeo & Juliet on Wednesday were not the usual champagne crowd. Attracted by lower ticket prices or by plain curiosity, these were ordinary Joes and Joannas filling the stalls. Never mind that MacMillan's 1960s production has been seen in London 300 times before; this house was charged with anticipation. It really was a people's night out.

Sylvie Guillem is not dancing every night, mark you. And Sylvie Guillem was what elevated Wednesday into the realm of the unforgettable. Among ballerinas Mlle Guillem is a nonpareil. God gave her a body put together differently from other dancers'. But it's the intelligence with which she employs that spun-glass physique, that enigmatic moue of a little girl's face, that makes her Juliet more than a match for the finest film or stage interpretation. The almost scientific naturalism of her acting (the adolescent pouts and shrugs, the impatient little movements) and the acute symbolism of MacMillan's steps combine to make a creature we can utterly believe in, for all of ballet's arch conventions.

Her Romeo, Jonathan Cope, was especially ardent in this performance. His piercing and obsessive gaze, from a full 60 feet away during the crowded Capulet ball, seemed to melt all obstacles in its path, and for an entire scene we ignored the scores of expensively costumed court dancers just to focus on that interplay of two faces. And the gorgeous abandon Guillem achieves in the bedroom scene - oh-so-nearly-explicit sexual gestures writ large in soaring, floating and lunging lifts - depend on 100 per cent trust in Cope's alert response and rock-steadiness. That he can supply all this and sport such magnificent thighs is the marvel of this particular pairing.

An unexpected plus of the new venue, apart from cheap drinks at the interval, is a huge orchestral pit. The large aperture means much more sound comes out, which might be a problem for opera (drowns the singers) but is a boon for ballet. Never mind the grand fortissimi of trombones in Prokofiev. The Russian conductor, Victor Fedotov, almost triggered a corporate heart attack with the National Anthem. If there were ever any cobwebs at the Royal Ballet, moving house has blown them away. Ignore doom-mongering reports of poor advance sales. The new crowd buys its tickets on the night.

The fact that most of MacMillan's ballets belong to the Royal Ballet means they're safe from the predations of the all-male Ballets Trockadero de Monte Carlo. Balanchine, his American equivalent if you like, gets sent up something rotten. In Go-Barocco - a skit on the great choreographer's "neo-neo-classical style" - sleek-limbed ballerbimbos form sweet interlocking daisy chains and skip about like the Three Graces, occasionally tying themselves in granny knots. The parody is as subtle as it is physically adroit, and the result is very funny indeed. Technical prowess reaches a peak in the Grand Pas Classique, when the ballerina - a glitter-ball in pointe shoes - mistakenly twirls the male danseur above her head. The Trocks have clearly struck a chord with the British audience. How could a bunch of gay New Yorkers know we enjoy subverting tradition even more than we love tradition itself?

'Romeo & Juliet': Labatt's Apollo, W6 (0171 304 4000), to Sat. The Trocks return to the Peacock, WC2 (0171 314 8800) in Jan.