Daniel Barenboim's Beethoven Piano Sonata Cycle is proving almost as much of a challenge for his critics as for himself. Still only halfway through this stupendous series, already widely recognised as one of the musical events of the year, if not the decade, Barenboim has us all ransacking the lexicon of superlatives in a feeble attempt to do it justice.

Older, wiser heads are shaken thoughtfully and the Beethoven performances of Schnabel, Rubinstein and Richter invoked, but such historic moments are not strictly comparable, and those of us at the Festival Hall in February 2008 as Barenboim's magisterial cycle unfolds are in no doubt that we are witnessing something monumental and unique: an event that is historic precisely because it is unrepeatable.

Barenboim's approach to the Beethoven canon is not a Classical one. As though to assert his Romantic credentials, he initiated Op27 No1 in E flat in a whisper several grades lower than pianissimo.

Second subjects are always strongly projected, too, in both tempo and character, just as Wagner insisted they should be.

There was a memorable Wagnerian moment also in the elegiac Largo e mesto Op10 No3 in D: "empty and desolate the sea" was the line recalled in this depth-plumbing reading of one of the great expressive expanses of early Beethoven.

The very end of the D major Sonata brought a special touch of genius: that mysterious rising and falling chromatic scale was executed with a touch so exquisite - aided by the kind of veiled pedalling in which Barenboim excels - as to open a window onto a new century (the 19th) of musical expression: a century of half-lights, shadows and intense subjectivity.

The E minor Sonata Op90 was a study in such half-lights: subtle, evocative and magically beautiful. If drama was relegated to the sidelines here, it returned to the fray for the final Waldstein Sonata. But even now it was ultimately transfigured into a halo of tintinnabulatory trills.

As has become the custom, the entire hall was minutes later on its feet, acknowledging the greatness of what it had experienced.

• Cycle continues until Sunday. Information: 0871 663 2500.