how good is it really at discerning etc
There are quite a few settings for AF, tracking, recognition, etc. With all of the settings in the middle of the ranges (defaults), and "human" tracking, it is very, very good. As a face you are tracking gets obscured by other activity in the frame (or the head turns away), it snaps back when the face reappears. Other faces don't seem to interfere with the face you are tracking. I am using BBF, with separate buttons for AF and subject recognition. Disclaimer: I am not convinced that my custom setup is what I intended— there are a lot of options! But at worst (my setting are stupid), it works better than anything on any camera I have ever used.
I have not tried registering a face, where (purportedly) it will stick to just that face.