How much of Drupal 8 will rely on or be compatible with Symfony? Now that Drupal 8 is being redeveloped with Symfony, and we are just about to go into the code freeze so that early adopters can start playing with it, what will it affect?

Will we be able to write modules and themes like writing bundles? Should we all start brushing up on or learning Twig?
Symfony, as in full stack Symfony, we aren't to be compatible with.

We are using Symfony components so many concepts used there will be useful.