It started off as a fork of d7 (really very early d8) but it's since diverged into it's own thing. It still has a core that is d7 compatible, so migrating is very simple (you can import a d7 database and it just works), but the theming layer is different. It's placed an emphasis on usability and modernization.

It's definitely a good choice for migrating from d7, and could also be a great choice for a new site for someone who enjoys working on a classic drupal stack.

The people behind it are great, but the community right now is kind of small.