Eclipse is a really great editor, which I prefer to use, but the GUI design tools for Eclipse are lacking. On the other hand, NetBeans works really well for GUI design.
For me using linked source folders works quite well.
I build the GUIs in independent NetBeans projects - if they need some simple classes or interfaces, I use the "link source" (right click on project in NetBeans, choose properties), to include these in the NetBeans project.
My main projects are in eclipse. Here I again use the link source feature to link to the NetBeans project (right click on project in eclipse, select "build path", then "link source").
EDIT (Thx to Milhous :) ): in both projects in eclipse and NetBeans further all required JAR files need to be added to the build path (also the libraries added by NetBeans: eg beansbinding-1.2.1.jar, appframework-1.0.3.jar swing-worker-1.1.jar, ...)
Now the GUI classes can be reused in eclipse. Also leads to need to have GUI and logic classes quite decoupled, which can be nothing bad.