The Shack is not great literature, so it will not change anyone's mind. However, it does contain great theology. If you are already religious and are looking for a way to express Christianity free from its corruption by power, The Shack is worth checking out!
Some good passages-
Expressing the biggest mistake within mainstream Protestantism:
pg. 66 - Nobody wanted God in a box, just in a book.
pg. 123 - "Once you have a hierarchy you need rules to protect and administer it, and then you need law and the enforcement of the rules, and you end up with some kind of chain of command or a system of order that destroys relationship rather than promote it... Hierarchy imposes laws and rules and you end up missing the wonder of relationship that we intended for you."
"Authority, as you usually think of it, is merely the excuse the strong use to make others conform to what they want."
"Isn't it helpful in keeping people from fighting endlessly or getting hurt?"
"Sometimes," Papa continued. "But in a selfish world it is also used to inflict great harm."
"But don't you use it to restrain evil?"
"We carefully respect your choices, so we work within your systems even while we seek to free you from them."
pg. 189 - "People are tenacious when it comes to the treasure of their imaginary independence. They hoard and hold their sickness with a firm grip. They find their identity and worth in their brokenness and guard it with every ounce of strength they have. No wonder grace has such little attraction. In that sense you have tried to lock the door of your heart from the inside."
pg. 182 - Again Jesus stopped. "Those who love me come from every system that exists. They were Buddhists or Mormos, Baptists or Muslims, Democrats, Republicans, and many who don't vote or are not part of any Sunday morning or religious institutions. I ahve followers who were murderers and many who were self-righteous. Some are bankers and bookies, Americans and Iraqis, Jews and Palestinians. I have no desire to make them Christian, 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."
"Does that mean," asked Mack, "that all roads will lead to you?"
"Not at all," smiled Jesus as he reached for the door handle to the shop. "Most roads don't lead anywhere. What it does mean is that I will travel any road to find you."