I read The Shack as a book discussion group assignment (said group consisting of theologically conservative pastors), and I can understand why the book is upsetting to many--whether because of the subject matter (the murder of a child) or because of some of the book's theological implications.
However, I think many are missing the point that the book is a parable, not a doctrinal treatise. I found reading the book to be an incredibly moving experience, and have (cautiously) recommended it to those in my circle of acquaintance whom I think would be able to digest its message.
I do find it interesting that so many evangelical Christians have had such a negative visceral reaction to this novel. This is especially remarkable in light of the fact that, by and large, evangelical Christianity has embraced C.S. Lewis' Chronicles of Narnia--this despite the fact that Lewis wove animism, Greek philosophy, and Roman mythology into his parabolic communication of Christian truth. Not to put too fine a point on it, but is it any more offensive to present God in the form of an African-American woman who cooks scones, than it is to present God's Son as an animal which imparts blessing to children by licking them in the face? (Incidentally, I thoroughly enjoyed the Narnian Chronicles as well.)
My recommendation: If you want theology, read the works of theologians. (Watch out for Sproul and other 5-pointers, though.) If you want an emotionally moving parable which hammers home the importance of a personal relationship with God, then buy and read--and be touched by--The Shack.