I'm actually surprised to see how many browsers support this: http://caniuse.com/#search=flex
It's a tempting apple. It could be nice for controlled enviroments, like iPad, Nexus and smartphones but for the open web it will simply not work. The support on IE is really sad, the 10 version is "partially" and the rest more or less also in the limit.
Independently of this particular grid system, I really think pure CSS grid where you add classes to the markup systems should be retired in favor of things like http://susy.oddbird.net where you define columns in the CSS itself.
Much like my grid system - FlexGRID - http://volumethemes.com/flexgrid
The focus of FlexGRID however is on equal column heights, and cross-browser support, with a fallback right down to IE8.
The columns are nice, but I'm not sure you need an abstraction of the flex properties into separate CSS classes.
Really useful. Thanks.