Surprising to see more than 30% developers voting "No" despite the great support. I can only assume from this that IE is still a issue for many developers.
imo, it's about time we stop supporting the older IE since Microsoft themselves have announced end of support for IE10 and lower.
This said, If your audience uses IE11 and above, there is no reason not to use flexbox. Flexbox spec was developed to fix the problems with CSS layouts, It's a much easier and cleaner.
The hashnode as you see, is built completely using flexbox and we do support IE11. 🙂
Extra: Here's another good discussion on the topic. Also, use flexbox but try not to get addicted to it. 😛