I acquired these discs as each set was released, so I owned the "full set" before the complete box set came out.
I've also had the opportunity of hearing Lewis play all five of the Beethoven piano concertos at live performances, with different orchestras and conductors at a number of venues.
I guess my starting point in evaluating his whole survey is that no single pianist can claim to offer the full story in every single sonata. If any criticism of Lewis is justified, it is that he avoids excess - and in the process the results in dramatic music may seem a tad "tame". Against that, I would add that at no point does his playing seem to me to be anything other than intelligent and above all musical.
Apart from individual discs of Beethoven sonatas by a large number of pianists, I own three sets - Emil Gilels on DG (sadly not quite complete), Artur Schnabel, and most recently Paul Lewis. Schabel may not be the "best" pianist technically but in terms of understanding what these works are about, he remains peerless. Gilels remains my favourite because of his unique tone and blend of drama and lyricism. Lewis seems to me to tread a middle course and I would be surprised if his interpretations did not develop still further with age and maturity: the current set remains for me a very enjoyable and satisfying listening experience, recorded with great naturalness and presence.