We have been doing Swing development for nearly the past 10 years. There are some nice GUI builders available (e.g. JFormDesigner), but all restrict us too much in different kinds.
For example, we have a lot of components without public no-arg constructor (e.g. a JTable subclass which requires the model in the constructor) or we have component factories.
Desktop applications usually have to be obfuscated. Obfuscation very easily breaks user interfaces created with a GUI designer or requires much work to avoid obfuscating such classes.
Another often happening case is that, for example, a panel should only contain some components depending on some condition. Simply hiding them would make the GUI look bad; they rather should not be added instead. I never found a GUI editor which provides this flexibility and even if there would be one, it would be so hard to use, that I definitely would be faster with good old Java code.