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.