I just don't think they have arrived at their final destination. Here is a recent question from Karl Young on the subject and my answer, as recorded in the IDL newsgroup. I should note that other people on the newsgroup took issue with my characterization and find the GUI Builder helpful. You should definitely try it for yourself before coming to any conclusion. No one I know builds GUI's just for the fun of it.
Most people build applications.
The GUI Builder is decidedly NOT an application builder, although I think this is RSI's intention for it, eventually. So, to give you an example. You open the thing up and build your nice interface. The API is slick, very Visual Basic looking, and quite easy to use. Your expectations are rising.