Let me deal briefly with the bullhorn guys...
Their main arguments are: God, the Father is portrayed as an African-American woman, and the Trinity's relationship with Itself(?) is unscriptural. I say 'Meh' to both of them, and let me tell you why. William P. Young never intended The Shack to be critiqued by theologians. He simply wrote it for his adult kids to show them God's character. So what if I didn't agree totally with parts of Young's theology? To judge this book based on points of doctrine is to miss the point of story completely!
One last rant if I may... How come so many Christians are quick to use secular movies/books to share the gospel (The Matrix comes to mind), but because a 'Christian' book is controversial, they condemn it as heresy?
But I digress.
The Shack is a fictional story which taught me (more than any teaching book BTW), about the love of God, why He allows suffering, and how especially fond of me He is (and if it sounds arrogant, you need to read the book ;-).
This is one of those rare books which, from beginning to end, is totally unpredictable... and emotionally exhausting. A warning to fathers of daughters: This book will mess you up big time, and the first-third is incredibly hard to read. But be encouraged, I do really believe that fathers of daughters will be the most impacted by this book.
In conclusion, and if you possibly can... I wholeheartedly recommend that you take a couple of days off work, hike up to a shack somewhere, read this book and thank God for His goodness. Then come home, give your kids (and wife) a huge hug and kiss, and thank God for all His goodness.