I'm sorry, I don't understand the question. Could you rephrase, please?
So do you implement the mechanic but not tell your players (which sounds complicated) or do you like the concept and would want to implement it, but don't?Me personally, I enjoy having books like UA lying around. Comes in handy for making adjudications sometimes. I use it as a "secret" GM mechanics system sometimes.
As for facing, it is one of my favorite. As a practitioner of combat irl I love the realism it introduces into a game that was based on a tabletop strategy game anyway. I was sad that we forgot, through the editions, where it had come from. Too often I have players take advantage of 360 degree awareness. As I like to be visceral with descriptions and they tend to interpret every word I say literally in game mechanics terms, it's become quite the lifesaver.
So overall the book for me is prophetic. My players would have me burned at the stake if they knew I said that. So lets keep this little secret between us...

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.