With Glade + Python 3 I was chucking out GUI code faster than the Visual Studio + Win Forms because there was no compile step. No over engineered MVVM ala C#/WPF for a simple app - load a glade layout file, hook up callbacks - bam everything works together. The MVC is already there because the content pipeline (model) is already separated and views are defined in glade files. I had a prototype functional by the end of the day - it would take me that much just to choose which tools to use if I went Javascript client/Python server.
Seriously - couldn't believe how fast you could get stuff done with it - everything works as expected, you get all the controls you need - no need to learn random stuff like that qtscript, no build process (this is actually huge !), insanely fast iteration (faster than web-dev). I always thought of GTK like some obscure C gui lib but the Python wrapper makes it very usable.
My only complaint so far is that Glade seems to be getting progressively slower as you remove/add stuff so you need to restart it from time to time (eg. once per hour) but it starts instantly so it's not really an issue.
Note : I haven't yet tried to get it running on Windows or OSX since we all use Linux internally.
Anyone that has ever used RAD tooling for GUI applications can easily see how the HTML/CSS/JavaScript combo tooling is still miles behind from a 90's desktop developer experience.
If you're not used to writing desktops apps, yes it will take longer.