Design Patterns was originally published in 1995 and is now on its 27th reprint.
It is the seminal work in Object Oriented programming. The authors have collectively made major contributions to every aspect of computer science and software development. Heck, the book is still not available in soft cover.
It is well organized and greatly informative. The writing style is clear and all but novice programmers should have no problem getting through the book.
But if you are buying the book to learn Java or C#, work mostly on database and web applications or are only going to buy one book on the subject, this may not be the best choice.
We really need a second edition from these leaders of OO programming.
The book is still 100% accurate and correct. But its contemporary audience was probably other computer scientists and experienced programmers who wanted to learn the emerging model. Accordingly, the book's code samples are in C++ and Smalltalk. The authors usually show the application of each pattern by solving problems from the GUI application development world. In 1995, designing portable windowing systems was probably the hottest project around (the Wintel world was still on 3.11 remember).
Thanks in no small part to Design Patterns, developers have tackled the challenges of windowing. Now, developers are probably more focused on the Internet, database portability and web services. They are using new OO languages like Java and C# (and C++).
An audience trying to work through those problems with those languages may find the book just slightly out of reach. Or at least somewhat indirect: you have to make up your own Java code samples and figure out how some of the patterns that solve GUI portability might facilitate database portability.