I finished The Shack last night. First off, let me say that I thought the story was good and I do not intend to spoil anything about the actual story. However, I think that some people might be misled into thinking that the story was based on a real-life occurrence of the author's good friend.
As the author explains here, the main character in this book does not really exist. In the Foreward and Afterward of the book, the author talks about this Mack guy like he is a real person and I feel that he was intentionally trying to trick readers into thinking that this really happened.
The book does have some interesting views involving focusing on a personal relationship with Jesus and viewing bad circumstances in a different light. So, if you're looking for an inspirational story based on a man's confrontation with God, then I'd recommend the book.