I'm quite happy with the modern flexbox syntax. I did a lot of work with XUL and xulrunner years ago - it was slick having things automatically flow into place, but the syntax (and syntax of the derivative box-flex CSS properties), but it seemed to require very heavy markup to get things "right".
Flexie [1] appears to provide a polyfill version of flexbox. Has anyone used it in a major production site? What was your experience? There were a few polyfill-type projects for the older syntax, but I never got them to work very well for anything but the most basic layouts.