The FAQs on the website say this isn’t for prototyping? Some of the videos and screenshots look like the forms are interactive, showing different states etc. Is the output html not actually interactive? If it was, I think it could be a real winner.
I’ve used tools like Axure, Just-in-mind and even Adobe Muse for designing and developing prototypes for web apps, all have their ups and downs, especially when considering responsive layout. Most of the other tools around just allow for clickable hotspots and maybe gestures. This seems to be the real gap in the market.