It is a full-width theme containing a fluid grid system, max width of the grid is defined in the theme settings form while the full width wrappers will always take the full window width.
Full On inherits all of Sasson goodness like HTML5 clean and light markup, Responsive layout for mobile compatibility, Sass compiler and Compass built in, Semantic grid system and more.
It is not a complete theme ready for production, but it is not a base theme as well, I would call it an advanced starter-kit, it has some styling in it but most of the styling is left for you.
The main task accomplished here is that this theme overrides Sasson's original layout and ships a full width wrapper for all page elements (header, footer etc.).
Other than that, Full On comes with a basic color scheme that mimics Bartik's look to make you feel at home (all colors are defined in _colors.scss and can be easily overridden).
Full On overrides Sasson's page.tpl.php and two of the layout partials (stylesheets), It is a more of a proof-of-concept and an example for sub-theming Sasson then a separate project, but if it also fit your needs, everybody wins.