Google is hosting 4.4.3 images for the Nexus 5, both Nexus 7s, Nexus 10, and Nexus 4. These are full images that must be manually applied and will erase everything on your device. Users interested in not losing their data should wait for the OTA update, which is slowly rolling out to devices now.
T-Mobile has a changelog for the update, which is wildly descriptive:
- Security enhancements
- Various bug fixes
Apparently, the update fixes problems with the data and Bluetooth connectivity. The security fix prevents apps from changing the launcher shortcuts of other apps. The Nexus 5 gets camera fixes for the auto-focus function and better battery life.