I have played around with Flexbox a bit during the last few days. The browser support acctually really surprised me. I guess the lacking support in IE is what stops people from using it in real life.
According the caniuse.com/flexbox around 80% of all people using the web would be able to render flexbox. Nothing near perfect, but still enough for some specific usecases I would say.
CSS-tricks have an awesome guide to all the flexbox properties. It really covers most of it with some nice graphics.
I have heard some things about it being computationally heavy in some browsers. Other than that, it is the perfect solution in many ways. CSS might not be such a pain in the future after all. I am working on some more advanced layouts with flexbox. I might post some of that here later on.