Nice chunk from T3’s iPhone X review:
It suddenly dawns on your that your face has indeed unlocked the phone. It’s a strange moment. We hadn’t had to make any effort to unlock it. And that’s what Face ID is supposed to be like.
Also, this was interesting:
It isn’t the barrier that you imagine it could be, though we did find it a little weird to have to make sure our face was available when using Apple Pay! Now, Samsung’s facial recognition (to unlock the Note 8 and S8) is close to Face ID. We were impressed at the ease at which it unlocks the Note 8 recently. But it doesn’t do anything else. Because Face ID is so baked into iOS it means that you rarely need your passcode. And it’s not just about unlocking your phone with the iPhone X, it’s about Apple Pay, App Store payments and more.
Face ID is baked into the OS, in just the same way as Touch ID is on earlier models.