This isn't perfect by any means but it's still amazing to watch in real-time as a computer figures out how to map one person's facial expressions onto another person's face. This isn't replacing a face with another face. This is replacing emotions on a face with emotions from a different face -- much more complicated it seems and much harder to make look realistic.
Although the teeth and eyes sometimes still weird me out (it's that uncanny valley poking its head out), the movements do look pretty good. I could see this being used in movies and video games very soon.