I found out about this books existence in the last week, and within 24 hours had ordered a copy. It didn't disappoint.
As an enterprise application admin, I look to squeeze every last drop of performance from our JVMs, and this book is a superb reference to all aspects of JVM performance.
My one (minor) gripe is the focus on SPARC hardware and the Glassfish app server in certain sections.
Given one of the authors works on Glassfish, and is an employee of Oracle, I suppose this is fair. However, I would have loved some coverage of alternative application servers, and correspondingly increased coverage on the modern developments in processor architectures outside of SPARC.
One other gripe I have is that JDK 7 is hardly referenced at all, despite its architectural and performance underpinnings being publicly documented during the books gestation over the last 3 years.
For the most part, many of the documented performance improvements for 7 have been back-ported to various 6 updates, but I believe JDK 7 deserves a distinct section at some point in the book.
If you're a Glassfish and/or SPARC user, this is most certainly a 5-star book, but for X86 and/or Tomcat users, this will form a part of a wider library.
For that reason, I'm giving it 4 stars.
All in all, an extraordinarily useful addition to my performance reference library alongside High Performance MySQL: Optimization, Backups, Replication, and More