These recordings are as close as anything comes to definitive. Many people mention Boulez's ability to X-ray a score and yes - every detail is beautifully realized in these recordings. But most important: Boulez like no other brings the form to life. Boulez never over-emphasize the music. Everything is played effortless without pressure, and at the same time, as a composer himself, he knows exactly when a instrumental role is important and when it should be more in the background. He knows when a little cresc. will do the whole difference and so on. A good example is the recording of The Rite. I have around 35 recordings in my collection (including three of Boulez's older recordings), but this is the one I return to again and again, simply because it's the only one, where I can feel the work as a unified whole, and not a sequence of tableaux.
The box contains the three pre-war ballets, a disc with "symphonies" (the Symphony of Psalm is electrifying and one of the most beautiful interpretations of a work, I have heard), some of the early symphonic works and a couple of disc's with older recordings of chamber-works and songs - wonderful candy to the ear and many overlooked gems.
The only bad thing to say about this "complete" set is all the Stravinsky-works Boulez hasn't recorded yet. How great it would be to hear his version of Les Noces, Oedipus Rex, Agon or Requiem Canticles. Well - He's only eithy-five and has never been better. There is still hope.
Even if you have one or more of these recordings already: Go for it.