Wrote this to a friend just now, who was asking:
I read the Shack last year and had mixed reactions. It moved me, and does draw one into the Trinity in some positive ways. It does some good things with the problem of evil. But it distorts God's attributes according to modern "touchy-feely" ideas too much. Makes God into one big empathizer with us in our suffering, instead of the one who conquers evil. Also interesting God the Father couldn't be a man.
I wouldn't say it's sinful to read it, just be careful of the way a book like this can re-fashion and distort our view of God, even unintentionally, while it claims to be drawing us TO Him.