Can I use and Flexbox support
Since there is a good amount of support for Flexbox across most browsers, but some like IE or Opera are lagging behind, Why would you use Flexbox if you know some browsers will not support it? For example currently, IE browser 9 and 10 do not support Flexbox so this means a very very large audience will be left out or disappointed in the website's design. To me, this seems like a big waste of time developing if you know people will not see an attractive well designed website.
Kevin Gates14,668 Points
What Kyle said, plus Microsoft has ended support for IE 9 and 10. So unless your target market is likely using outdated PCs and software (such as an AARP website), then likely you can move ahead and use Flexbox without issue and if needed add that additional disclaimer about needing to use a modern browser.
Sidenote: if more websites encouraged people to move to newer browsers, this would actually be a good thing as it would bring more security to the users and decrease the risk of fraudulent or malicious behavior against those users.