When I first learned about this book I found it an exciting way to codify and categorize various OO techniques - I wouldn't say it introduced any new design/implementation strategies, but it gave us all a common terminology for what we'd been doing all these years. Since then, I suppose the terminology has largely entered the programming lexicon so it's useful for all OO programmers to be familiar with these concepts and terms.
However, we are way, way overdue for a new edition, one written using C++11/14 or modern Java for the examples. The C++98-based examples really date this book - lines and lines of code to illustrate what you'd do with a bit of STL in modern C++. The patterns themselves are still relevant, but I hope no one is taking the code examples too seriously.