While we don't have any specifics from T-Mobile regarding the update, it can be assumed that it brings the standard KitKat features such as immersive mode, transparent status bar, lock screen upgrades, and cloud printing support.
Rick is also reporting that his update went smoothly, without any apps going missing and that his launcher settings and layout remained intact.
If you haven't yet received the 496MB over-the-air (OTA) update, you can force a manual check by heading into Settings -> General -> About device -> Software update.
Have you received your update? Everything running smoothly? Let us know in the comments.