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).
One technique is, alongside flex, to use Sass/LESS/Stylus mixins (or just CSS classes), using float and widths: IE8/9 use those, they're ignored if flexbox is supported. It takes a little work, and you have to limit yourself somewhat, but it's reasonably robust. Effectively, you describe a float-based grid, but include all necessary flex-based declarations. Alternatively, you describe a display:table based grid, but that has fairly serious downsides.
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.
Thank you; people like you help change the status quo.
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.