This book is not just an exegesis to GoF design patterns book, it's much more than that.
This book teaches the right use and right meaning of object oriented programming concepts. For example, in academics, inheritance is taught as a way to extend the behaviour of base class. While this is true from implementation perspective, this narrow definition will lead you to make improper usage of inheritance. It's not uncommon to see developers creating a sub-class for each special case, thereby creating huge inheritance trees and creating maintenance nightmares.
I benefitted a lot from this book, if you aren't a very experienced object oriented designer, you'll benefit from it too. If there's just one thing to take from this book, it's "Commonality and Variability Analysis".