I don't really care for Shostakovich, but this is his 100th anniversary, so I suppose I should make some comments of some sort or another.
Boulez is wrong. "Third-pressing Mahler" is a rash and insensitive judgment that has more to do with his postmodernist agenda than any other external factors. However, I think people tend to place his works solidly in the Stalinist milieu. That's fine, but that doesn't mean every note, bar, and movement has something to do with Stalin, the terrors, and the other world events of the middle age of the Soviet Union.
Shostakovich should be allowed to stand on his own merits, and that's where the trouble starts for me. I just don't think he's all that great. Certainly, he deserves a place in the repertoire, but to turn him into some sort of Stalinist Loge, commenting with ironic aside after ironic aside about the events in Moscow.