This is actually pretty cool, one step closer to making Google Docs a worthy competitor to Office. If they combine this with an Access-type database app, it could be a great tool for businesses. Although using the interface builder brings back memories of VB...
The UI designer from old-school VB the best part about it. Obviously, the language part was crap. I think that forms designer still represents a rapid UI development environment that many languages/environments have not reached.
I typically do more webapps than anything else, so I have gotten used to designing my own UIs from scratch. I don't mind as it helps keep things very lean... but I'd love to use something like Sencha's app designer thing. I just can't justify the cost currently.