From what I can see so far it looks quite interesting and may be a great solution for rapid prototyping. I have not had much time to really mess around with it but it does look pretty sound. I have a feeling that major customizations would not work too well.
The basic package is free and includes one project which is enough to get in and play around. Check it out if you’re interested in a responsive layout rapid prototyping tool.
What is Jetstrap?
Jetstrap is a 100% web-based interface building tool for Twitter Bootstrap. No software to download, just log in and build. Your work is accessible from anywhere.
Who is it for?
Built for developers, designers, and idea-people alike, Jetstrap helps you get awesome websites up and running fast, with less work and digging through docs.
Why you’ll love it
Life is too short to dig through docs when you just want to build. Grab snippets of clean markup and quickly use complicated components. Take off faster!