GUIDE looks quite impressive in its screencast. I've never used a GUI builder, well not since Visual Basic, many years ago, so I can't really compare. However, GUIDE has been built by friends of mine, so I'm sure it's good!
"We're looking for developers to take GUIDE out for a spin and help us shape it up for general release. You can use GUIDE unregistered with some feature restrictions, or fill in your email address and we'll send you a licence valid for 30 days. You can request a new licence as often you need. If you have any problems with this form please contact us at firstname.lastname@example.org."