We recently used flexbox in production, but only for really simple things like equal height columns. At the time of writing I think it's a bad idea to use flexbox for managing grids and other 'big' things.
When I need to do matching height stuff in browsers that don't like flexbox I use this tiny bjit of vanilla JS to measure and apply the min-height for me http://codepen.io/tomhodgins/pen/pjmKNj