I think it's very challenging to create a middle-reader Holocaust story that isn't either "too much, too conflicting, it hurts!" or a little old hat--though to be fair, if you go through years of Hebrew school (as I did) you'll get a lot more Holocaust story exposure than most of Yolen's potential audience: that's the context of my personal review.
Not mind-blowing. Powerful, certainly; I did stay up reading in bed until I had finished the book. Overall, predictable story of modern-day girl going back in time to realize the scope of the atrocities at not-Auschwitz-but-an-unnamed-literary-equivalent.
Though the title and the brief conversation/narration that explain the titular "Devil's Arithmetic" are outstanding, devastating, perfect.