I started collecting this cycle piece by piece only a year ago and completed it in August. Now DG releases it in a big, cheap box. I feel a bit cheated, but the individual discs have de la Grange's splendid notes, so I won't complain too much...
Regarding the performances, I have to say that most of them are splendid, as they are insightful, interesting, thought-provoking, well-played and well-recorded. Granted, I won't agree with everything Boulez does, but at least he has something new to say about this often-played music. His approach to Mahler as the precursor of the Second Viennese School is, at any rate, very cogent, and I respect his Mahler cycle very much for that.
I won't comment on every performance here because I've written them down some time ago here:
http://topear.wordpress.com/2013/09/05/a-consideration-of-pierre-boulezs-mahler-cycle/ I think it's a great palate-cleanser to those used to Bernstein, Tennstedt, or in fact everyone else. Most highly recommended.