These are just my preliminary impressions, having received this set this morning, although some of the recordings were already familiar.
As I write this, Byron Janis is playing the Schumann piano concerto and very well too. This is one of the later recordings made by Mercury (1962) using 35mm optical film instead of magnetic tape and the sound quality is really good. I believe that the film has suffered less deterioration in the ensuing 50 years than tape, with its tendency to "print through" and flake.
Overall, on the CDs listened to so far, the sound has that "brightness" that I have always associated with American recordings of this period. The ear soon acclimatises but the initial slight shock lingers in the mind - try disc 18 (Dorati, LSO, Khachaturian's Ganeh suite) to see what I mean.
Sound quality of these recordings, made on purely analogue equipment with valve (tube) amplification, is still very good. I'm playing it back on valve-based amplification to get the full effect.
Of course, the most striking characteristic of this set is its amazing value - just over £1.50 each for 51 of some of the finest recordings ever made, often by great artists. You can't go wrong at this price.