Ok, here's my few thoughts regarding "The Shack" which has generated enormous response from the Christian community, and some very polorizing reviews as well - from mighty endorsements (Eugene Peterson) to mighty denunciations (Mark Driscoll).
1) Just as "This Present Darkness" helped people "understand" spiritual warfare, so "The Shack" seems to help people "understand" God especially through deep pain & suffering. Both books help to visualize and bring to life the invisible realm, of spiritual warfare, and what God is like, respectively. That is the power and attraction of these books.
2) Just as "This Present Darkness" was not a doctrine book, so I do not take "The Shack" to be a doctrine book. However, both books have doctrinal and theological trajectories, that if taken to it's conclusion could be troubling, if not outright heretical.
3) Speaking now to just "The Shack" I believe the author Wm Paul Young has employed some literary devices, pictures, and dialog to exaggerate some features and truths about God and spirituality in order to accentuate them, and bring out their qualities more vividly. There is some truth in them. However, if Mr. Young were to freeze his story line and depiction into doctrine and theology, then I would dismiss the book out of hand because it would clearly be a case of truth out balance; turning minors into majors.
For e.g. God the father is depicted as a "she" in the form of a black woman. Obviously this is unconventional, non-traditional and grabs your attention. The bible never depicts God the father in female form in any way, but it does refer to God the father as having "motherly" affections. (see Nu. 11:12, Is. 40:11, Is 49:15, 1 Thes. 2:7). So to personify some of God's "motherly / female" qualities as a "African-American" woman to illustrate a point is ok (or even to make God a bit more "personal and soft," hence accessible may be ok), but if you turn it into a doctrine, that God is a "she;" then you've crossed over into heresy, or goddess worship as Pastor Mark Driscoll says.
Another e.g. Mr. Young portrays the trinity as having no hierarchy, just a circle of relationship. There is clearly a spirit of mutual submission, good will, love and unity that is amazing in the trinity. It could appear completely relational and egalitarian, but in fact, the trinity is not this. The reason the trinity works in such perfect harmony is because each person in the trinity knows his place in the circle. There is a trinitarian hierachy and authority structure which is the key to stability and safety for all humanity and eternity.
The list goes on. Case after case, the author likes to exaggerate certain truths to make a point...
p. 182 "I have no desire to make them Christian (referring to Buddhist, Muslims, Mormons, etc) but I do want to join them in their transformation into sons and daughters of my Papa, into my brothers and sisters, into my 'Beloved.'" The author is big on relationship; it's all about relationship, so he pens this quoted sentence to strain / highlight this truth about relationship, almost to the point of breaking. On the otherhand, just when you think he's adovacting a universalism of salvation, his dialog says this: "Does that mean all road will lead to you? Not at all...most roads don't lead anywhere. What it does mean is that I will travel any road to find you." (A statement any Calvinist would rejoice over).
p. 209ff - Chapter 15 could almost be taken as New Agey aural and "the light within" kind teaching. He speaks of emotions and thoughts as being reflected in differents colors of light that we can't see, but if God were to open our eyes, we would be able to. Now, in heaven, I do believe we'll see things in dimensions we've never seen (eye has not seen - I cor 2:9); but again, the thought is written such a way, people could easily misinterpret it.
Mr. Young's emphasis on forgiveness, healing, working through pain are wonderful, and in some regards this book represents his journey, his magnum opus of what he's learned.
Conclusion: Incredibly well written with compelling story line and dialog that is first rate. If read as a novel / fiction which points to and highlights some things that are not emphasized enough in the Christian faith, I think it has some merit, and helps seekers access God in a helpful way. However, if the story line is turned into doctrine or lines of theology, then it would be heretical. Following up a reader with some solid bible study would do well to put the book in its proper perspective and balance. I do think Mr. Young himself could alleviate a lot of controversy by penning a companion piece stating his theological positions so it can be settled if there was something more definitive he's trying to get across. (I've read his blog, windrumors.com, but nothing definitive is there).