I don't think flex is a good idea if you still have to support IE: http://caniuse.com/#search=flex . It seems to have a number of issues and needs a few workarounds. Imho, flex is quite practical if you do not care for IE any more and instead do what @leaverou said in here AMA: Only support the last two versions and no IE. Of course, for customers or in the industry that rule might not be applicable.
And IE will no longer receive updates fixing those problems, as fixes might break existing websites and there will never be new IE version any more (we have Edge now, so that makes sense)