The most notable new additions are probably Multi-Window, the handy multitasking feature that allows you to run two apps in split-screen view, and Samsung KNOX 2.0, the award-winning security solution. Apart from these, the KitKat update also brings a wide range of small changes in the user interface – some widgets have been removed and certain apps have been renamed. A lot of existing performance and connectivity issues have been resolved, too.
Verizon's Samsung Galaxy S4 mini (SCH-I435) is also receiving the sugary KitKat treatment via a firmware update, which is rolling under the KOT49H.I435VRUBND5 build number. Apart from the KitKat goodies, Verizon has also treated its S4 mini to an additional feature or two. For starters, Google Hangouts with SMS support is now preloaded, the stock Samsung Messaging app has received a nifty Font Size menu, and KNOX 2.0 has been thrown in for good measure, as well. A number of performance and stability issues have also been fixed, most notably the Wi-Fi connectivity bug and the force-closing of Google's Voice Search.
As usual, you should take into account that the updates might not hit your Galaxy S III or S4 mini right away. Hopefully, you won't have to wait much.
source: Verizon (1), (2) via Droid Life