You didn't really have to wait that long, I used it on a site a year ago (Autumn 2013) and managed IE8 support along with FF, Chrome, IE8+ with a combination of the 'old' flexbox and the new (+ a couple of IE hacks).
All current hacks available are pretty fragile. Best solution is a separate set of rules scoped to .no-flexbox (or .flexbox if you're going the other way). It's a massive pain in the arse, tbh.
Anyone aware of any large sites using Flexbox that has non-tech users, e.g. Netflix or a major ecommerce site? I ask because I'd like to see any presentations or write-ups those guys may have written about their experience.