My company recently went all in with flexbox, and you should too. We had the same concerns, but even IE 9 is 4 years old at this point, and we decided we shouldn't be held back because of it. If a client wants IE9 support, they can pay extra.

I have run into a few IE10 gotchas, but they're all solvable. IE10 just uses an older version of the spec.

and IE itself will be cut loose from MSFT in a few months.