These realizations by the Leipzeg String Quartet (with added musicians for the sextet) are outstanding in every way.
Schoenberg's 3rd string quartet has always been one of my favorites: this is what the genre is all about! I will say here, that while the Leipzeg SQ here relaxes the music just a little to let it breathe, giving it a little extended playing time--which I usually wholly approve of--I slightly prefer the LaSalle String Quartet's realization on Deutsche Grammophon for its speed and trenchant, cutting sur-reality.
That said, however, this disc is highly, highly recommended.