Mahler is very much about two things. Brilliant musical and orchestral details, and deep emotions. The former are all in the score, but the latter has (in my opinion) to be shared by the conductor to a certain extent. The reading by Pierre Boulez is exactly what one would expect it to be. The details are nealy perfect (only Karajan outshines Boulez in some places) and the tempos seems very "right". Especially in the last movement, which he does uncommonly fast, he manages to bring the music to life without adding anything. On other occations, notably inte first movement, his lack of anything that isn't inte score is irritatingly obvious. There is no suffering, only sounds. The bottom line is that although this might not be the only Mahler 6 to have, it is a good place to start with it's immaculate playing and sonics, and a "middle of the road"-reading. Then you can always add a Barbirolli or a Tennstedt for a reading with all the emotion and power you can shake a stick at, but without any traces of finesse.