A bit disappointing for me, I expected a little more.
From my point of view it is a bit too focused on Reflection, which is something to avoid.
The book was written before Java 1.5 (Tiger) was released, so some ot the issues it covers are outdated, even though it tries to describe some of the "future" Tiger's features.
Anyway, it presents some tricks and tips on the internal workings of the JVM that are not commonly known (at least I was not aware of them), so it can be worthwhile to take a look at this book.