The RSC's much-heralded King Lear arrives at the wide open spaces of the New Theatre, giving Brits the chance to see Ian McKellen in one of the theatre's greatest roles. It is worth the wait.

Lear is an uncomfortable play. It deals with large themes - the arrogance of power, stupidity of war, cruelty of torture and corruption of law (how far from the world we inhabit!) — as well as the intimate problems of aging and the family. It puts these together and challenges our most basic assumptions about life itself. But its grand scale and problematic text make Lear a very hard drama to bring off.

Ian McKellen plays his part magnificently. His Lear is a King-priest who strides onto the stage in a storm of organ notes, calling down blessings on us all. But all too soon this regality is shown to be rigidity and the blessings turn to curses that rebound and destroy him. McKellen depicts Lear's decline with merciless clarity and power. His cursing of Goneril (Frances Barber) is genuinely terrifying, evoking sympathy for her even as she plots against him. He also differentiates Lear's madnesses convincingly — from stormy rage to surreal rambling – while his gentle recovery and deathbed scenes would melt the coldest heart. Whichever of Lear's many moods he is portraying when McKellen is on stage he commands it.

But Lear is more than a star vehicle and sequence of brilliant scenes and unfortunately other aspects of the production lack the lustre McKellen brings to his role.

While Christopher Oram's set is simple and evocative – a colonnaded balcony, swathed in red, disappears off to the right – the action is mysteriously updated to a nineteenth century Cossack kingdom, making the play into a sort of Sisters Karamazov. Stephanie Arditti's costumes are gorgeous: Goneril's black ball gown, as she advances to destroy her father, has more menace than a cruise missile, but Cordelia's white one makes her look like a skittish teenager who's cranky from staying up too late. When she later dons a stylish military greatcoat to lead the army of France, well, I'd have deserted on the spot.

Trevor Nunn's direction is naggingly inconsistent, ranging from the expressionism of the opening ritual through food-based realism to sitcom-style wisecracking. Julian Harries' henpecked Albany looks more like Hugh Grant's older brother than a pagan chieftain, while Philip Winchester cannot drag his evil Edmund too far from away from pantomime villain.

Along the way we take in a Cossack dance for this is the RSC, and must have their singing and dancing and sword play. While these episodes are always consummately done, they can also clog the rhythm. So can an uncritical approach to the text: the chillingly credible sadist Cornwall (Guy Williams) appears not to have met the disguised Kent before, though a few minutes later Kent (Jonathan Hyde) tells Lear that they did. More distracting are the antics of Poor Tom (Ben Meyjes) and the Fool (Sylvester McCoy). It may be heretical to say it, but judicious cutting would make their contributions even more effective.

The Fool looks particularly odd surrounded by Cossack soldiers, eerily reminiscent of an earlier role of this actor's which I am too discreet to mention, and it is hardly surprising when they decide to hang him (though it is surprising that one of them appears to be the King of France who married Cordelia about an hour beforehand). This hanging they do on stage, just before the interval, so that the audience has to walk out leaving the poor chap dangling there.

But for most viewers the play will be carried by McKellen. Though it is to be hoped they will not overlook the more than able support of Barber and Hyde and Monica Dolan's Regan. But William Gaunt's Gloucester is also magnificent. If the play's most violent moment is his blinding, then its most daring wordless metaphor is when the sightless man sits alone and defenceless while a battle rages all around him. But its most daring line is "Never, never, never, never, never!" and this McKellen draws out into a lament of echoing inter-stellar emptiness.

If this Lear turns out to be something of a mixed bag, it is still unmissable.

The cast for this production which is booking at the New London Theatre (Drury Lane, London, WC2 Box Office: 0870 890 0141 12th January 2008 features Romola Garai, Frances Barber, Jonathan Hyde, William Gaunt, Monica Dolan
With: Ben Addis, Adam Booth, Zoe Boyle, Russell Byrne, Naomi Capron, Richard Goulding, Julian Harries, John Heffernan, Peter Hinton, Melanie Jessop, Gerald Kyd , Seymour Matthews, Sylvester Mccoy, Ben Meyjes, David Weston, Guy Williams, Philip Winchester. For full production details see Les Gutman's review of the BAM production after the LA review.