Steer clear of the +Samsung USA Galaxy S4 +AT&T #KitKat update.
Yeah it has some nice features and improvements, but it trades those for a major Los of performance. My device downloaded the update automatically once I was on wifi, then updated automatically with no way to stop it after I postponed it a few times.
It deleted or disabled a bunch of my apps and widgets, especially any sideloaded ones. However when I redownloaded them, most of them had their settings and data were still intact.
Apps are now denied access to (almost all of) the external microSD card.
Lag is increased, performance and battery optimization are decreased.
The lock screen occasionally stays completely black.
And there's new bloatware to boot.
If you're going to be on wifi, I suggest you backup up your device, app list, and important app data before it updates itself.