Hennessy and Patterson had a good base to start from and this new edition is even better. It adds a CD-ROM with additional articles covering topics such as vector processors, interconnect networks and even a survey of instruction sets. There is even a search engine on the CD that covers the book and the CD-only contents.
The book has been updated so it covers the latest computer architectures like the 64-bit AMD Opteron as well as those from Sun, Intel and other major vendors. Each chapter is an excellent mix of theory and examination of existing hardware. Each chapter also has a Fallacies and Pitfalls section that typically includes real-world examples. Even experienced developers will find something new in these.
The chapters address major topics starting with Fundamentals of Computer Design and moving onto parallelism, including a chapter dedicated to multiprocessor architectures. I especially like the detail of the Memory Hierarchy Design chapter that delved into virtual machine considerations and an in depth examination of the AMD Opteron memory architecture.
The appendices make up a major chunk of the book. Appendix B covers instruction set principles. It would make a suitable semester course all by itself. The comparison of different architectures provides the same kind of useful insight to different design approaches as presented in the rest of the book.
I highly recommend this book for those learning about computer architecture or those wanting to understand architectures that differ from those they are currently using. It does an excellent job of covering most of the major architectural approaches employed today.