The Shack really does read like a kind-of-pilgrim's-progress and a kind-of-systematic-theology. The story is compelling. Yet, at the same time, Young, who is a dad and a guy who holds a regular job out in the rat race, dares to tackle HUGE subjects like the nature of evil, the nature of the Trinity, the problem of pain, the atonement, forgiveness, justice, and the meaning of love.
The fact that Young uses fiction to tackle theological issues is one of the great things about the book. Having gone through its first print and now getting serious attention from Christians and seekers across the world, it is going to cause lots of folks to begin talking about some pretty serious theology. Hurray!
At the same time, one of the down sides is that Young, at times, makes a pretty big mess of some of the BIG stuff. Namely, The Sovereignty of God is very limited in Young's theology and God is often portrayed as someone who loves us very much but just can't do anything to help us at times because he does not want to interfere with our choices. Additionally, he tends to see hierarchies as wrong and rejects any kind of hierarchy within the Trinity and subsequently in human relationships, choosing to believe that total equality is the answer to many relational difficulties. According to the whole of salvation History, God has made it pretty clear that he can and does interrupt our choice making, because he loves us. In fact, he has been known to wage war against his own people because of their refusal to choose Him- He is that jealous for us (See the book of Hosea or Ezekiel 16, for instance). Likewise, the Bible clearly shows Jesus willingly submitting all things to the father and it shows beauty in many hierarchies including the created order, the relationship between God, angels, man, animals, and in the human scheme between man and woman, and man and government.
However, these problems are not a reason to avoid the book. Instead, they are reason to read and think and grapple with big things. Essentially, I would say the book is worth reading mostly because Young really shows us the power of God's Love, as Father, and he does a great job getting us to feel God's love through the power of forgiveness and the beauty of grace. Read it with your mind turned on and, if you have "daddy issues" keep the tissues close.