The Mariinsky is probably the only company that could stage the four operas of Wagner's Ring on consecutive nights, casting solely from within their permanent ensemble. Yet, while the scale of that achievement is impressive, it's impossible to pretend that their production is anything other than a flawed and uneven cycle.

The music is not uniformly exceptional, but in this production - masterminded in Wagnerian fashion by Valery Gergiev with his designer, George Tsypin - it was not just the inadequacy of many singers but the failure to confront the Ring's potentially stunning theatrical effects that was deeply disappointing. However beautiful Gleb Filshtinsky's Aurora Borealis lighting, when the ring of fire is conveyed by dancers with twisted filigree of day-glo pink on their heads, it's hardly the stuff of spectacle. Too often, design details were simply naive; that was consistent with the concept of a Russian Ring taken back to its primordial roots, but the disparity between elaborate festooning and crude oversimplification was risible. And although the permutations of the four lumbering figures framing the whole epic grew less irritating, as they gained or lost heads and decorative appendages, the multi-purpose stumps stuck out like glowing sore thumbs to the bitter end.

Larissa Diadkova as Fricka in Die Walküre and then as Waltraute in Götterdämmerung was outstanding; Mlada Khudoley's Sieglinde and Leonid Zakhozhaev's Siegfried showed great potential. Olga Sergeyeva's Brünnhilde grew into a formidable presence. However, three different singers could not add up to a single towering Wotan.

The Mariinsky orchestra rose memorably to Wagner's passion. If exhilaration and exhaustion alone prove the point, then something of Cardiff's Bayreuth aspiration was realised.