One thing seems to be clear: Having the face slapped creates intense feelings in both, the one who is slapped and the one who does the slapping. It is therefore that opinions about this subject vary very much - as it can also bee seen by looking at the ansers in this thread - even among us: people who generally enjoy slapping or being slapped.

It is due to this intensity of feelings that I think that it is interesting, but when done, it has to be done with great care due to possible injuries and (even more important9 due to possible emotional pain. Face slapping can hurt!
If however both partners are comfortable with each other, feeling each other's mood and thoughts it can be an interesting game, due to this psychological aspect.

Personally I would only slap the face if I had had this person as a partner for long time already, knowing all her dreams and fears and of course only if this game would first of all create positive emotions in her.