This really seems like a winner. I need to take a few hours in a coming weekend to give it a try, to have enough familiarity with it to have in the back of my mind for a project that it would work with. It really seems too good to be true.
Their web site’s home page does a good job of boiling it down:
Jo was made for apps, not websites.
Your HTML5 app lives in a single page and Jo helps you create a native-like app experience. Sure, there’s some DOM manipulation going on in there, but you don’t have to worry about it unless you want to. Build your app with JavaScript, tweak some CSS and call it a day.