I must warn you that this review is based on only 2 of the 9 CDs of this box. The reason is simple: that I bought the albums separately and stopped buying after the second.
I short, I had the misfortune of shelling out a lot of money to audition this set. But seeing that Sonatas 21, 23, 30, 31 and 32 belong among the greatest of Beethoven's works, my reasons for not completing the purchase might give you a basis for your own assessment.
So this reviews the discs containing No. 11, 21 and 23, as well as 30, 31 and 32.
Gerhard Oppitz is undoubtedly a fine pianist, at home in the tradition of classical pianism, although clearly influenced by modern ideas too. His readings of these sonatas command respect; they are technically accomplished, well balanced from beginning to end, you never feel that his grasp of any of these sonatas is episodic. He knows exactly where he is going.
His approach is cool - by which I mean he sounds somewhat detached, there is a hardly measurable emotional involvement to be discerned. Some collectors will be put off by this. It means climaxes are not driven by a powerful ethical impulse, which seems to be the essence of Beethoven's artistic profile. Rather Oppitz is contend to play them louder, as required by the score, but this means that a deep conviction of their necessity is missing. Similarly the more gentle moments in the music convey nothing like a sense of yearning.
No. 11 and the Waldstein Sonata illustrates both these point unmistakably. No. 11 has no conspicuous emotional depths to plumb and so fares outstandingly well under his hands. The C major tonality of No. 21 elicits from Oppitz a very "white" approach: I've never seen him play, but I can imagine that not a muscle moved in his face. The 1st movement is very virtuosic, but the rigorously maintained pulse makes little allowance for air to come into proceedings. The 2nd movement played like this sounds altogether perfunctory, every note is played with the same value, not the slightest touch of rubato hints at an interpretive concern.
When we turn to the Appassionata, this non-involvement of the human being in his music turns into a disaster. This sonata is riven by tempests on the outer movements which reflect one of the blackest states of the soul in all art; and especially the last pages of the Finale plunge us into a maelstrom of unutterable despair. The hearer who is not shocked to the marrow by this cataclysm should probably stay away from this sonata. This is not a pleasantry. But none of this touched Oppitz - and you should compare especially these last pages with Gilels, whose reading is one of the glories of the gramophone. In Oppitz's playing there is no differentiation between the 1st and 3rd movement to let you understand the enormous tensions having been built up and exploding in the end. In fact he is so unmoved by it that I can't tell of any change of approach from the 2nd to the 3rd movement - it's just faster and louder.
There is no change of approach to the later sonatas (30-32). One could easily put an argument that the qualitative difference of the works should be reflected in the pianist's manner. But it doesn't happen.
It may be one reason why op. 109 is the best of this clutch. As a work, it seems almost a prototype for op. 111, but lacks the sheer depth of the latter. Judged on its own merits, Oppitz's reading is clean, energetic and, in the slow movement, free from sentimentality. But you need only hear Backhaus, whose approach on the face of it is similar, to note the subtleties which Oppitz overlooks, tiny inflections that "humanise" the tone of voice. Op. 111, on the contrary, suffers badly from the pianist's unwillingness to delve into this work's meaning. This is not to be played, but to be experienced; it is not just roar and thunder followed by a long-winded Arietta with boogie-woogie intermezzo. This sonata is a supreme testimony to the human capacity for suffering and grief and consolation. Nothing like it on exhibit here. I will single out Kovacevich as a rival who exhibits a profound understanding of it; and in this comparison Oppitz comes off a very bad loser.
To summarise: Oppitz plays the sonatas; and he plays them exceptionally well; but he plays the score without any discernible effort at giving us an "interpretation" of the music. I personally find this distressing.
I also dislike the sound recording - you should certainly sample it extensively. The mikes are some distance away, and the recording site very reverberant. At any of the many excited moments in these sonatas, the effect is a noisy blur and end up sounding like a momentary cacophony. The more you hear them (this is my fourth sampling of the albums), the more unpleasant it becomes.
As mentioned, I was discouraged from going further down the track. On the strength of these two albums, Oppitz has nothing to offer a listener to whom Beethoven represents one of the musical summits of Western Civilisation.