I have built my own small company site (working as freelancer) on Drupal 8: http://www.stefan-korn.de. (Sorry only in German language for the moment)
I started with Beta 3 and had a real hard time to get it updated. Finally I have managed to bring it to Beta 11. But needed to more or less recreate the site. I know that it is clear there is no supported upgrade path between beta versions, but really did not think it would be that hard ...
I have only worked with Drupal 8 Core, put in some libraries manually (masonry for responsive grid, magnific-popup as lightbox for images) and used some custom templates.
Also views had major changes between beta3 and beta11 regarding classes that are used. In beta11 default classes that you have always known from views are mostly removed for cleaning up html output. While this is a reasonable approach, it can be hard for the guys that got used to those classes and relied on them for CSS styling.
I think Drupal 8 is more a revolution than an evolution regarding Drupal 7, so it is clear that there will be changes that themers and developers need to adapt to. I appreciate the great work of the Drupal community and think that Drupal 8 can be another great milestone in the drupal history. The concept of totally redefining major parts of the system is impressive and shows that Drupal is totally alive and kicking.