It takes a lot for an IDE to catch my interest. I have been working with the Eclipse Visual Editor, and have been completely blown away. Even in it's prerelease state, it is one of the finest GUI designer's I have used.
Installation is a snap, just download 3 eclipse plugins, extract them, and restart eclipse. The designer works great with any layout (Grid, GridBag, and even null layout for a true WYSIWYG designer).
What totally blew my mind though was the complete flexability of the tool. My hand coded GUI forms were very close to the generated code done by eclipse so, with VERY MINIMAL coding I was able to have my hand coded application pulled into the GUI editor. Plain and simple, this is the most amazing visual designer out there.