Drufony is made of four different projects developed during a few weekends and holidays in October and December 2013.
As explained elsewhere, the Drupal fork is branched three times.
7.x-pimple replaces just the Drupal bootstrap with an extensible Pimple container. From my testing, it is a drop-in replacement for Drupal core with no side effects.
7.x-autoload replaces the Drupal 7 class registry with Composer autoloading. This is not a drop-in replacement for Drupal core, but it demonstrates how you can subclass the Pimple container to alter Drupal's bootstrap routines.
7.x-symfony contains all of the changes required to make Drupal 7 function inside Symfony. The changes from core are minimal.
Overall, the differences between Drupal 7 and my Symfony fork are relatively small. As of 27 December 2013, there are 324 additions and 99 deletions in 21 files.
Benjamin Doherty (bangpound) is a freelance, open-source Drupal and Symfony developer.