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.
Cloud Garden makes a GUI editor called Jigloo that is quite nice if you are into that sort of thing (and the price is very, very reasonable). If that's all that's missing for you from Eclipse, I'd recommend that you take a look. Netbeans does a ton of stuff with source code that you aren't allowed to edit, etc...
One other thing that I will mention: I have used GUI editors like Matisse and Jigloo for super rapid prototyping. However, within 3 or 4 iterations, I always find myself dropping back to hand coding the layouts. I also find that when I'm doing rapid prototyping, I am almost always more productive when I change the layout manager to absolute and just place components. Once the design starts the gel, implementing the design by hand coding using a good layout manager (I strongly recommend MiG Layout) is pretty easy, and gives much better results.
I know that dragging and dropping a GUI layout is really enticing - but MiG Layout is incredibly productive for hand wiring GUIs, and I suspect that almost any developer will be more productive within a week going down that path.