For sheer depth and profundity, and an all-encompassing view of every detail of this massively complex work, I have yet to hear the equal of this performer. His harpsichord is an old original newly restored (Rückert). It sounds magnificent and is magnificently recorded. As is the case with all hand-crafted instrument, there are occasional hiccups - these things were not machines after all - but they endear them to your heart, because in those few moments you are made aware of the human craft and human art that are involved.
This album deserves a grand review, a piece by piece elaboration. But the buyer will only wish to know what he or she gets for their money - and the outlay in this case is quite significant! What you get is a player who knows everything about the 48 that anyone could know; he has performed the work many times throughout the world; he has a capacious imagination and deep insight; and his scholarly researchers are impressive (especially in the Art of Fugue, of which I will speak some other time); and finally he is not shy of either letting himself go (in terms of virtuosity) or immerse himself to the full in the several deeply reflective and even grief-stricken Preludes. The rewards offered by this album exceed the price. This is an investment not for a few weeks' entertainment, but for life.
I note that the 48 have been split into two albums. It is logical, considering there are actually two works here. In practice it means that buyers have the choice, if they wish, of buying the Well-tempered Clavier on its own. It is entirely excusable on the understanding that the pieces of this work are on the whole shorter and the overall level of original inspiration seems to be ever so slightly higher than in the Preludes and Fugues of the second part. However, the latter includes some pieces that plumb greater depths than anything in the Clavier, which would be distinct loss to a devoted Bach friend. But this is a decision you must make for yourself.
I refrain from dealing with comparisons. Frankly i believe this set is not directly in competition with any other version; it stands above competition. His pupil Ton Koopman made a complete recording which is commendable; Kirkpatrick on Archive is also very good; Helmut Walcha made one version on harpsichord (EMI) and another on the organ (Archive); and there are many other versions which soon degenerate into mere names. Therefore I say to you, in all sincerity: Buy this first, even if it means sacrificing your budget for the month! For whereas you can easily compile a list of 20 pianists who might all lay claim to being the "greatest" of the 20th century, with harpsichordists you would be scratching to find more than two of such calibre. And Leonhardt is the younger of those two.