Frankly-- and I say this as a major Braxophile-- the Mosaic is not essential. While taking the work as a whole works on the level of "look what he managed to do with major label bucks in just 7 years", I think taking the albums on their own is more representative of Braxton's vision. Not to mention there's equally vital work from this period on Moers, Hat Hut, Leo, Black Saint, Muse & Freedom. The LPs have decent sonics, are easy to find, have great covers, and, above all else, contain exhaustive notes. While it's not Mosaic's policy to include liner notes in their sets, I think the lack here borders on insulting. While I'm fine not having Ira Gitler's 500th bland rundown on a Lee Morgan date reprinted yet again, Anthony goes into strenuous detail explaining his musical systems on these albums and to have the music without this goes explicitly against Braxton's intent. Mike Heffley's notes in the Mosaic are fine but hardly relevatory. Lastly, the way the music is divided is slightly confusing and breaks up some of the records.
I don't mean to complain overmuch. I'm thrilled Mosaic put this out and I'm glad I bought it. But for the purposes of answering your question, this is what I think.