It describes handy techniques to create a testable and flexible software system. Honestly, it changed the way I write in an object-oriented style. Ruby is used for demonstration purpose.
For beginners:
It is an excellent guide for people who just started to learn object-oriented programming, but some of the highlighted problems might be unfamiliar.
For experienced:
It is a fantastic resource to update your object-oriented programming. It worth to read the short book at least to make sure that you do everything right.