Twenty Two years since the book's publication it remains incredibly relevant. In the beginning I was surprised the discussion in 1994 was this high level. The preface and introduction are awesome. For example, there was one discussion about dynamically typed languages versus statically typed lanugages... I was not even aware this was being discussed in 1994.
In certain situations you see how this book changed the way the field of computer science developed. Before the writing of the book the authors originally called the Singleton pattern the Solitaire pattern. They changed it last minute (explained in the Conclusion) from Solitaire to Singleton, and that is a major part of why everybody calls it Singleton today.
Some people may have an issue with the age of book. When you read the introduction, they mention that C++ and Smalltalk are cutting edge programming languages. I know C++ pretty well, but I have never used Smalltalk. What I learned from the book was how Smalltalk was fundamental to creating the MVC (Model-View-Controller) framework. In a lot of places the authors point out situations where C++ programmers would implement a pattern one way, and Smalltalk programmers might use the pattern another way.
The book's examples are mostly about text writing programs, windowing, and drawing. These examples fit well for the patterns. You can also see how the current state of programming was much different. Text editors were creating huge innovations back then.
This book requires sophistication as a programmer. It will be a challenging book for pretty much anyone to understand completely. You need to have familiarity with the word choice as well. The authors assume you are well versed in their language. The glossary was pretty good in this book, I would recommend taking a look before you start.
The progression of the book is excellent. There is a lengthy introduction before getting to the patterns. This helps put the entire book in context and prepares you for the challenge to come. Each pattern is unique in subtle ways that the authors explain masterfully.
One hundred years from now this book will still work. The patterns are fundamental to software design itself. I wish most authors were this bold.