Certain versions of the Galaxy S4 had seen the update earlier, but it wasn't until Tuesday that U.S. GS4s were able to access it.
The updates are known to be currently rolling out to AT&T, Verizon Wireless, and Sprint GS4s. As is typical with these things, they tend to be staggered, so don't be concerned if your device doesn't see the ROM right away. The changes appear consistent across carriers (Verizon PDF, here).
Chief among the changes is app to SD support. This will allow end users to shift apps from the device's internal memory to an SD card, freely up device storage.
Other changes are said to also include Samsung Knox security software and a new locked bootloader, which isn't going to go over well with rooted users. There are also more minor changes.
As of this time, we haven't heard of a T-Mobile update. All of the carriers appear to be delivering the ROM update over-the-air (OTA).