Very useful. For the heck of it, Ive stuck a couple of pics here that show you what it looks like in the WND editor we used.
Pic 1 - Window
Pic 2 - Button
The useful thing is the Heirarchy view as you can see which thing (button, drop list etc) fits into each window and so the program takes care of the whole parent/child window system.
Im still trying to re-write it to upload it for wider use - the shots there are from my edited version that so far has translated and works on machines other than a developers one
Its a full WYSIWYG editor as it uses all the images from the mapped images folder and your string file, plus you can simply move stuff aroud within the window (that 'grid') and drag n drop elements. You can throw together a window in no time.
The problem is that because its an internal tool only, its designed only to work with internal builds. Ive got that copy running with the final version of Generals, but GUIEdit only reads STR files, not CSF, and it will only read 256x256 texture pages/INI files not the 512x512 ones that the public version has. Still, a bit more work and I should have it licked.