The book has faced so much criticism... But I think peoples opinions on the book speak louder of their own mental states than the contents of the book. It can be used to become 'evil' and manipulate and use people for your own gain or it can be used to help liberate people who are being manipulated, by seeing past the tricks of their abusers ... It really depends on how you use the information in the book and your intentions.
For me personally I see the book as a way to gain a better understanding of people in power and while some things seem far fetched when I think deeply about a lot of the stories he referenced they are things that have happened to myself and others around me, by teachers, politicians, narcissists, bullies, fake friends etc...