First off, Boulez is not everyone's cup of tea. His early works in particular are very craggy, complicated, and highly serialized without any really discernible "themes" to guide you unless you follow along with a score (and even then, you are drawn to it more for its complexity than for anything else). I find some of his works to be near unlistenable sometimes -- they often give me a headache because you have to pay attention to everything all the time or else it's just a bunch of noise. And sometimes even still, it's just noise.
As others will mention time and time again, Boulez is constantly updating, withdrawing, and re-submitting his works. So this set shouldn't be called his "Complete Works" because they're not -- Boulez himself admits this. He chooses to omit Polyphonie X and Poesie pour pouvoir. It's more his "curated works" and even then, it's not complete. For example, the book included claims Boulez "finished" the missing movement IV for Livre pour quatuor in 2012, but that isn't included here -- a recording from 2000 represents Livre pour quatuor and obviously doesn't include IV. Why? I don't really know. Supposedly (according to Wikipedia, at least), 7 of the Notations have been orchestrated, though only 5 have ever been performed (or recorded). His health reportedly has been failing as of late (the man is 88, after all). So this set may well not only be what I call "curated" but also "definitive" for many years to come.
Most of the music here has been previously released, and it tends to get glowing reviews from reviewers because the only people who would buy Boulez's music are most likely admirers of his work. I am less effusive about his music, though his work from the 1970s onward I find to be more attractive -- he drops the hyper-serialism of his early days to take up the inclusion of electronics, spatial composition (which these CDs do not represent very well), and the use of unusual timbral combinations. The last 5 discs of the non-historic, non-interview CDs (discs 7 through 11) are just full of really striking works. Repons is a ravishing swirl from start to finish, where electronics, cimbalom, and a chamber orchestra happily meet. And the Derive and Anthemes pieces are brilliant, where you can see how the second version takes off from the first.
A few works are taken from concert archives and are live. The Livre pour cordes on Disc 7 is a live recording, and it suffers from an unfortunate audience member coughing up a lung. The Notations for orchestra and Derive 2 don't have the problem to the same extent.
The booklet included is quite extensive, as it describes each of the pieces in detail and includes texts of the sung works. But it is not a particularly idiomatic translation, so it can be hard to read and at times kind of weirdly written. But knowing the history behind each of the works is a fascinating glimpse into Boulez's "Work in Progress" philosophy.
Perhaps someday I will come to love La marteau sans maître in the way that the musical cognoscenti already do. As I work towards that, the rest of this set provides plenty of enjoyable music to listen to -- I just wouldn't listen to it every day. But if you have any interest in Boulez's music, this is a set to own.