I've been drooling at the opportunity to have a full Java2 stack on my TiBook and Mac OS X hasn't disappointed. Probably the greatest benefit is the range of options that a developer has when writing Java code on Mac OS X.
First, you can write 100% pure Java code which uses AWT or Swing and you get a great-looking UI. I've had hardly any issues downloading software that was impossible to run with Mac OS 9's MRJ 2.2.5, which runs great on Mac OS X.
Second, you can optionally add Mac OS-specific technologies to your apps by using Apple's JDirect Carbon-Java bindings. This allows you to really create Mac-style apps with Java.
Thirdly, since Java is my favorite programming language, I can use the Cocoa APIs and have an ultra-rapid development environment. I wrote a quick front-end tool for Ant in Cocoa-Java that's very full-featured and works and acts just like a first-class Macintosh application. It's ridiculously cool.
Of course the biggest draw is that you can run it on hardware that's not only tuned for the environment, but that supports features that you just can't have under Windows or Linux. When was the last time you felt really good about running Linux on a laptop. I know folks do it, but Apple's really been able to tune OS X for their machines and the consequence is speed and stability.
Mac OS X is a great place to develop Java!