Samsung definitely likes to take its time with these things but the update is now rolling out to non-carrier branded S4s according to SamMobile.
However the update is only for the international GT-I9505 LTE variants of the handset, meaning the US will have to hold tight for a little longer.
In terms of what's new with the Android 4.3 update there isn't too much for you to actually look at, but we're told that plenty of improvements have been made.
One significant addition is support for the Galaxy Gear smartwatch, while Samsung Wallet now also comes pre-loaded.
Other listed improvements listed by SamMobile include OpenGL 3.0 support, improved RAM management and TRIM support. Basically, it's a fair bit more finely tuned and therefore a tad faster.
There's no word yet on when carrier-locked S4s will be getting an over-the-air update, but we don't imagine it will be too long now.