The update was first rolled-out but was shortly found to be infested by a lot of bugs and was since pulled. Not after AT&T resumed seeding the fixed version of the update. And now the update has started making its way to other regions.
The update brings improvements to performance, the Galaxy S4 gets Galaxy Gear and Knox support along with Group Play 2.5 and more.
Users that have installed the buggy 4.3 update should shortly receive a small patch update through Samsung Kies. And the Galaxy S4 should receive Android KitKat soon as well so this isn't the last one.