From an Apple support document:
Even if you don’t enroll in Face ID, the TrueDepth camera intelligently activates to support attention aware features, like dimming the display if you aren’t looking at your iPhone or lowering the volume of alerts if you’re looking at your device. For example, when using Safari, your device will check to determine if you’re looking at your device and turns the screen off if you aren’t. If you don’t want to use these features, you can open Settings > General > Accessibility, and disable Attention Aware Features.
And from a security white paper published today (PDF):
To improve unlock performance and keep pace with the natural changes of your face and look, Face ID augments its stored mathematical representation over time. Upon successful unlock, Face ID may use the newly calculated mathematical representation — if its quality is sufficient — for a finite number of additional unlocks before that data is discarded. Conversely, if Face ID fails to recognize you, but the match quality is higher than a certain threshold and you immediately follow the failure by entering your passcode, Face ID takes another capture and augments its enrolled Face ID data with the newly calculated mathematical representation. This new Face ID data is discarded after a finite number of unlocks and if you stop matching against it. These augmentation processes allow Face ID to keep up with dramatic changes in your facial hair or makeup use, while minimizing false acceptance.
Apple also provides information in that white paper about a Face ID Diagnostics function that users can opt into, which will record all Face ID unlock attempts as images for seven days and can optionally be sent to Apple for analysis.
I have written here before that I have no idea whether Face ID is going to be good enough in most circumstances to replace Touch ID. Outside of the lucky Apple employees who are using an iPhone X as their regular carry device, nobody truly knows. But from everything I’ve seen in Apple’s documentation and everything I’ve heard from those who know about using Face ID on a daily basis, it’s the real deal for secure, reliable, and fast facial recognition.