Update (10.10.2013): Good news: according to recent tests, Flexbox layout isn’t slow any longer. Author’s comments about the performance of Flexbox refer to the original (legacy) flexbox that used display: box;. A head-to-head comparison of old vs. new syntax is available as well. — Ed.
When the mockups for the new Financial Times application hit our desks in mid-2012, we knew we had a real challenge on our hands. Many of us on the team (including me) swore that parts of interface would not be possible in HTML5.