According technical details provided in a podcast by Jon Siracusa, who detailed how the technology works as only Siracusa can about when the passcode is entered, the technology behind Face ID, does take additional facial shots which result in furthering the local, Face ID db. ...These incremental updates, if called upon by entering the passcode, is how Face ID learns whether one has grown a beard, changed hairstyle, etc.
Here's a link to Apple's
Face ID Security Guide But, Face ID works as advertised. Fast. Reliable.
I've had my X for 10 days (as of this writing), and Face ID has worked each and every time, regardless whether I'm wearing glasses (were not worn during initial set up), a hat, or hair is messed.
As result, during this initial usage period, I have not yet been required to enter my passcode; and I use my X throughout each day. The only time I have entered my passcode was after restarting my X; and entering one's passcode after a restart (or after 48 hours of device not being unlocked) is required just as it was for Touch ID.
Additionally, the process (lift X, glance - default setting - at screen while lifting, & swiping up from bottom of the X) is equally fast as using Touch ID. This process is fast enough that it feels as if there's no security procedure at all since I am not waiting to actually see the unlock icon, but simply glancing at the screen (when Lock Screen appears) & then, swiping up with my thumb.
Frankly, I already prefer Face ID over Touch ID, which I use every day on my iPad Air 2. Additionally, I now believe Face ID is the future for Apple's new products, and Apple will work towards supplanting Touch ID with Face ID.