I must say I like Glade a lot. It does have it's own set of troubles and bugs, but version 3.6.7 works quite well.
What I like the most about Glade is the ability to change GUI layout without making a single code change. As long as my widgets in GUI definition file keep their names and types, application will work exactly as before the change.
But even if you don't intend to use GtkBuilder in final version of your application, you still may benefit from it. This is especially true for packing widgets into GtkTable. One needs to be quite experienced to predict what the final outcome will be, but in Glade, you can see in real time how changes in packing parameters affect the layout.
And last, my favorite "feature": choose one builder file and run:
$ grep "<object" my.builder | less
What you get is one nice widget tree that can help you a lot when thinking about problematic parts of the GUI.
As for the big applications, Glade still may be used to develop them, but you'll probably want to split you GUI in more manageable parts (design dialogs as separate builder files for example). In my experience, big applications usually need quite a few custom widgets. And since implementing GtkBuildable interface is not the most important thing if you intend to use those widgets only in this application, Glade is out of the question.
Hope this info helps a bit.