This will not comment on the new translation, too much: having read in the early 1980s, the original acclaimed Ralph Manheim translation originally published in 1959, I can't pretend to remember that version clearly enough to compare the two, although my copious excited undergraduate notes in the margin bear witness to how struck I was with the language of whatever the original might be. All I can do is compare my original underlinings with the same passages in the new one.
Mitchell, to his credit and to my slight discomfort, adds an intriguing afterword about the differences between the two translations. He wants to make clear that there were things in the original manuscript that just couldn't be printed in 1959 (although not that many, as one of Grass's multitude of prodigious literary gifts is his ability to lyrically and cryptically euphemize without quite having to hand you the keys to the car -- Martin Amis comes to mind, among others, although I'm sure both Amis and Grass would point to Nabokov as the more unitary model in dressing up one's salaciousness). Sterne's The Life and Opinions of Tristram Shandy, Gentleman also comes to mind, since we hear the narrator's voice incessantly on every detail of existence preceding his birth, although he is not born until the end of chapter 3 ("Besides, the nurse had already cut my umbilical cord. There was nothing more to be done" in the original, substitute a semi-colon for the period after "cord" in the new translation. Can you hear me now? Mitchell seems to be saying.)
Or, for instance, in the original, "As always where the future of Poland is at stake, the Virgin Mary was in on the proceedings" has become "As always when the future of Poland was at stake, the Virgin Mary appeared in the crowds at these conflagrations," leading me easily to pick the former over the latter, going with easy colloquialism of Manheim over the anti-slanging of Mitchell. "Impersonating a meat-colored baby" is now "Impersonating a reddish blue baby." Little stuff like that. To Mitchell's credit, he does claim to have restored Grass's original long sentences in many places that Manheim had turned to a series of short sentences, softening some of Grass's rhapsodical, poetic stretches as a way of introducing him to a large American audienc. The long sentences as paragraphs are great stuff, and seem totally authentic to Grass's intent.
My notes of old go some distance in proving that, in 1981, I was struck by the forest, not the trees: I liked quotes, back then, stuff that could be plucked out and read aloud, as aphorisms. And there are certainl lots of them. But reading it now, relatively quickly, utterly sober, it is the sweep of the narrative that amazes me, and the intricate realistic sculpture of the landscape of Danzig, what Grass refers to as the "provincialism" of his novel, in his preface, where he also recalls wondering as a 29 year old why any self-respecting publisher would be interested in such a story steeped in a small-town locale like Danzig.