My first response is that you run jBuilder on OSX! You can run jBuilder 4 (and 5) on OS X, since it is itself a Java application. Also, a "Mac OS X-specific" version of jBuilder is due out any time now.
Also, you need to realize that there are huge differences between ProjectBuilder and jBuilder. ProjectBuilder is PRIMARILY for developing mac-centric applications (not always, but usually). I'm talking about Cocoa/Carbon/WebObjects development, as well as java-based-Cocoa apps.
jBuilder is ONLY for developing pure java apps. You will NOT be able to create java-based-Cocoa apps (unless the OSX version includes it, which is 99% doubtful).
Having said that, if you want to develop pure java apps, I have seen nothing better than jBuilder (definately *NOT* ProjectBuilder). Others are as good (Forte/VisualCafe), but not better than jBuilder. But please don't limit yourself to running it only on winders. At least run it on Linux!
Hope this helps. It really depends on what you want to develop. If you're considering developing for a pure winders environment, use VisualStudio. But I would not recommend doing that--I'm a pure java guy running jBuilder, and ProjectBuilder for WebObjects.
---gralem