Hi
I am not saying that flexbox will be ideal for everyone and all sites, IE9 users have already fallen below that of IE10 in all charts, but, (maybe that should be in capitals) like everything else with statistics, the user base for each browser varies depending on which set of statistics one looks at, and I have seen IE8 below 4% in one, and above 20% in another.
Flexbox, is I think a serious alternative to some current responsive design solutions I have seen, and even providing an alternate IE9 and below style sheet, (these browsers are now only used on desktops) is less trouble than using the 'not mentioned' alternatives for responsive layouts.
PZ