LAST TIME round it was Armani, this time M&S. Cosi fan tutte gets the People's Opera treatment - you'll have read the headlines. But when Jonathan Miller's classically chic Cosi returned on Tuesday with its cloth cut, literally, to the Royal Opera's straitened means, it was hard to tell which costumes were high-fashion as opposed to high-street. And frankly, the whole thing remains such a miracle of craft, it wouldn't matter if the cast wore dustbin-liners: they would still be stylish, still amaze you with the elegance and detail of a show that represents contemporary opera staging at its most refined.

Each tiny gesture is immaculate and calculated. Nobody crosses his legs without considering whether it should be left over right or right over left. And while that could be oppressive, it somehow isn't. There's unflagging freshness in the laid-back, laddish teamwork of the two male lovers, Simon Keenyside and Rainer Trost. Their women, Barbara Frittoli and Enkelejda Shkosa, are a delight. And so, as Alfonso and Despina, are Tom Allen and Inger Dam-Jensen.

You might quibble with the solo singing: Trost doesn't quite float the aerial phrasing of "Un'aura amorosa", Frittoli's "Come scoglio" could have a more solid foundation, and the lustre is largely gone from Allen's voice these days. But these are quibbles in the context of performances fine-tuned into a real ensemble by the raised left hand of Colin Davis: a conspicuous feature of the show.

In other ways, you might wish Davis wasn't quite so obviously involved: as usual, he sings along with the performers, turning trios into quartets. But the detail in his music-making matches Miller's scenic microsurgery. And his approach to Mozart is supremely civilised: a touch slow, but so cultivated that you savour every phrase.

The Devil has been all but cultivated out of meaningful existence in the modern world. The Church of England says there is no hell: official. Satanism - such a purposeful activity 500 years ago - has been reduced to midnught romps in Rickmansworth, where minor civil servants bear their bottoms to the moon. And give or take the odd screen comeback, the Devil doesn't even get much value from his Equity card, except in donnishly unfashionable plays by Bernard Shaw.

But he has one persistent refuge. Opera. And for these purposes, his vicar on earth is Samuel Ramey, the American bass whose identification with diabolic roles - in Gounod's Faust, Berlioz's Damnation, Offenbach's Hoffmann - is so complete, it would be worth examining his feet for signs of cloven hoofs.

This week he was at it again, in the Royal Opera's Barbican concert performances of Mefistofele: a maverick piece little known in Britain, where its composer, Arrigo Boito is remembered only as the librettist of Verdi's Otello and Falstaff, and widely dismissed as a musical white elephant.

But in its time (the 1860s-1870s) and place (Wagnerphile Italy) Mefistofele was considereddefinitive of the way ahead for modern opera. Written in romantically-inclined high seriousness, it set out to encompass the broad philosophical argument of Goethe's poem - as opposed to the love-interest with which Gounod's Faust contented itself. And in doing so, it covered so much ground that the original version of the opera took an infamous five hours - later reduced to a more practical, if sketchy, three.

The sketchiness of the story-telling is actually no problem for latter- day audiences, reared on Janacek and sufficiently grown-up to fill in the gaps for themselves. But the ambition of the piece - and Boito's tendency to be seduced by scale - is troublesome. It carries excess baggage, and not too muchtaste. In short, it's tosh. But glorious tosh. And at the Barbican I happily surrendered to its gaudy grandeur and the thrilling vehicle it offers to a dark but agile bass like Ramey.

It's a fun piece for the Faust too, sung by Richard Margison on this occasion in his massively-projected, matter-of-fact way, like a vocal slap across the face. And as Margareta isn't so big a role, it was combined here with the other object of Faust's affection, Helen of Troy, and sung by Nelly Miricioiu, whose bottom register is raw but rises to a glistening, statuesque top.

With the chorus (variously representing witches, warlocks, angels and penitents) whipped into deafening frenzy by a hyper-animated Bernard Haitink (whose feet left the ground several times during the witches' sabbath) it made the most outrageously enjoyable entertainment I've allowed myself all Lent. A stunning night: and cause to say again that despite the Royal Opera's struggle to maintain a decent stage life in its Wanderjahre, its track record so far on the concert platform is superb. Two cheers for that.

Covent Garden isn't the only homeless opera company in Europe at the moment. The Geneva Opera has been on the streets since last September, when the Grand Theatre closed for renovation. But the Swiss being the Swiss, they got their act together and built themselves a temporary theatre in the shell of a pumping station on the River Rhone. It's frill-less and functional, but elegant - and has the benefit of being a building that must have looked like an opera house even when it was separating sewage. Last weekend I caught a new production of Prokofiev's Betrothal in a Monastery there, sparely staged by Patrice Caurier and Moshe Leiser. And although it didn't sparkle with invention like the Guildhall School production a few years ago in London (probably the most accomplished student show I've ever seen), it took a clear line on the sort of comedy the piece requires (Prokofiev called it lyric-comedy: in other words, with greater textural richness than pure farce), and worked quite well.

The Scottish Opera Ariadne auf Naxos that opened in Glasgow on Wednesday works quite well too. But it's not so sure about the sort of comedy it deals in. Strauss set this opera-about-putting-on-an-opera in the home of the Richest Man in Vienna who, by implication, has more wealth than taste. So you'd expect some glittering vulgarity on stage. But the vulgarity should come in quotes - as a feature of the story, not the piece - and Martin Duncan's staging doesn't make that critical distinction.

The gilded campery of the design infects the whole show, smothering the pathos of the young Composer in the Prologue. It also earths the transcendence of the final Ariadne/Bacchus scene: a moment when the frame of the performance should dissolve outwards, recognising that this musical encounter has grown beyond containment in a rich man's salon. But Duncan does the opposite. He anchors all the Ariadne business to the Prologue, running the two together with no interval. And though that buys a coherence you don't always get from this Siamese twin of a score, it's at a price.

Anne Evans is engagingly put-out (rather than haughtily indifferent) in the title role, and there are pleasing contributions from Lisa Saffer (Zerbinetta), Diana Montague (the Composer) and David Stephenson (a matinee- idol Harlequin). But collectively, there isn't enough class in this production. Richard Armstrong's competent conducting doesn't fill the deficit.

'Cosi': Shaftesbury, WC2 (0171 304 4000), to Sat. 'Ariadne': Glasgow Theatre Royal (0141 332 9000), Tue,Thur & Sat; Edinburgh Festival Theatre (0131 529 600), 14, 16 &18 Apr.