This book will be of particular interest to you if you're using JBoss as your app server, as JMX forms the fundamental glue used to implement the JBoss server.
The book starts out with some nice introductory coverage for Standard MBeans, which are about the most dirt simple classes to code so I expected some good writing here. From there we start getting into Dynamic MBeans where the fun really begins. I was particularly interested in the implementation of ModelMBeans, which are dynamic MBeans that can map to a resource (like a printer for example) and provide automated attribute caching etc.
While the Dynamic MBean coverage was fairly adequate I was pretty disappointed with the ModelMBean coverage. Not enough time was spent describing all the Info classes and Descriptor attributes you need to implement. I spent a lot of time experimenting, reading the JSRs, and working through a subsequent ModelMBean implementation chapter to figure things out. There are also numerous bugs in the sample code for ModelMBeans.
The Chapter discussing the MBeanServer was a little light but reasonably well done. Enough to help you understand how it works which is all you need to program with anyway.
One sorely lacking area is JMX notifications. The interfaces are briefly mentioned, but there is little by way of sample code, or in depth discussion of the default JMX notifications. Since notifications are based on the AWT Event model you can find plenty of other source material for how the notification mechanism works, but I'd like to see more detail on the JMX standard messages etc.
I enjoyed the JMX standard services section. They were very instructive, and helped indirectly patch together some of the notifcication stuff.
The JBoss chapter is, of course, quite good with detailed information about how JBoss uses JMX, how everything fits together in JBoss with JMX. All in all it's a great case study of what you might do with JMX.
This book was a good introduction to JMX, but I felt it lacking in certain areas of detail. Since there are relatively few JMX books (in fact I think this might be the only one) it's not a waste of money, just not as insghtful as I'd hoped.