The Pierre Boulez fun will eventually stop, as there’s a limit to what is left for him to do, as certainly hardly any recordings (if any) that are not in circulation.
Joining the fun of last years, excellent, valuable and highly recommended Pierre Boulez: Complete Columbia Album Collection is the companion box from Deutsche Gramaphon, Complete 20th-Century Recordings on Deutsche Grammophon. There is quite a lot of overlap between these, of course, as Boulez re-recorded the bulk of this music for DG. Where this box stands out is with the recordings of music by Birtwistle, Messiaen and Ligeti, with a little Szymanowski tossed in if that’s your thing.
How does this compare in quality with the Columbia recordings? It’s an impossible choice: personally, I prefer his later Bartók, Schoenberg, Webern, and especially Ravel—these are my favorite Ravel readings. I listen to his Columbia Debussy, Berg, and Stravinsky more frequently. You can’t go wrong with either, but do you need both? If you don’t have the other box and need a cornerstone set of 20th century music, you want this box. Otherwise, ask your inner collector (FYI I will be resisting this box, because I have all of these recordings already except for the Szymanowski, and I’ll survive).