I have their set of Schnabel playing the Beethoven piano sonatas. The reason I purchased it is that I was in the middle of collecting the Naxos releases of this music, when they were deleted in North America because of a copyright dispute between Naxos and EMI. I live in Canada, and that settlement should only have affected the US market, but for some reason the recordings are now unavailable here, too.
Anyway, I picked up the Membran set to fill in the gaps resulting from the missing Naxos volumes. Compared to the Naxos transfers (by Mark Obert-Thorn), the sound quality of the Membran is significantly poorer to my ears. Though the Membran has less surface noise, this seems to be accomplished at the expense of the piano tone, which has a hollow, "sucked out" quality, and is much less immediate and present than on the Naxos discs. The Membran set also comes without any notes. Still, for US$16 for a ten CD set, I guess I can't complain too much. I just wish I could still get the Naxos'.