If you feel that Bach needs to be intimate, with scaled-down forces, in order to be effective, this definitely is not for you. Klemperer viewed Bach as a monumental, epic composer, and saw the Mass in B minor as not only Bach's greatest work, but the greatest musical creation of Western civilization. I tend to agree with this view; as Wilhelm Furtwangler put it, performing Bach's great choral works with small ensembles is like trying to turn Michelangelo's Moses into a desk trinket without making it appear absurd. In less capable hands such an approach would feel artificial, bloated even, but Klemperer had such a keen sense of form that one always feels the inner logic of the music. As an example, listen the Gloria in excelsis followed by Et in terra pax: in the transition, the powerful and majestic gloria transforms inexplicably but inevitably into the mysterious et in terra, and then gradually, ever so slowly, builds through the fugue to it's climax, even greater in proportions than in the gloria. Klemperer, like Furtwangler, understood that form is not only about differentiating the various themes that occur in a piece, but also placing them in their logical context. Indeed, with the German classics, especially Bach and Beethoven, it is not the themes themselves that carry the spiritual content of the piece, but rather the evolving context in which they are placed. If you compare this recording of the B minor mass to many more modern ones, you can immediately see how rare a gift this is. Most conductors are happy at merely executing the notes precisely, or at best giving a sense of the expression, but the greatest of the great were never happy with this. As Mahler put it, what is best in the music is not to be found in the notes.
As far as the performance quality, the soloists are beyond superb. I have never heard performances of the arias that can touch these. The Philharmonia orchestra is in top form, and the BBC choir, while occasionaly having balance or intonation problems, copes very well with the demands Klemperer places on it. The recorded sound is superb, as one would expect from the late 60's (I have never understood why so many people refuse to listen to non-digital recordings, as if the tiny bit of added clarity digital has to offer makes the performance suddenly worthwhile).
I have often wondered why there was such a difference between Karajan's two studio recordings of the B minor mass, from the 50's and the 70's. The first seems light, airy, and lyrical, while the latter is, put bluntly, overweight. Listening to this, it seems fairly clear to me that Karajan was trying to borrow from Klemperer's approach here: his lack of success is yet more evidence of Klemperer's sovereign command of this music. He only listened to one of his recordings for the pleasure of it: this was it.