They are ready to be used today, and im using this branch in my consulting work.
The caveats are:
-many components still need to be converted to variabled less
-theming is undocumented but you might gather an understanding from conversations on github issues
-the folder structure might shift around.
Currently user overrides are in /user/ which will be overwritten if you use a package updater like bower, this will most likely be in a directory like "user.example" in a future release which you will have to manually rename to avoid it being touched by updates to SUI (ala some CMS etc which include sample config files)