Flexbox is here to stay and its time to start using it. Flexbox has more than 95% support globally, which matches support for calc and viewport units. Bootstrap 4 will be supporting it and my new website will be Flexbox based as well :)
Oh and if you need older browser support … use Flexibility.
Bulma is a modern CSS framework based on Flexbox. It provides a nice set of core functionality and almost every kind component you might need to build a quick skeleton for your project.