I'd be hesitant to use that in prod code - realistically you can achive fairly graceful degradation without it as long as you're prepared to accept a non pixel perfect layout.
Also, from the issues v2.X that polyfill appears be broken in IE9 atm, which is the last one with zero flex going on. (Fun fact, our metrics have ie9 and 10 with comparable userbases - fairly standard large public facing site. 9 is fully unsupported and IE10 is no longer an active target for our QA team beyond basic layout and functionality)
I do generally use a less mixin to shim all the tweener syntax tho.