I've come close to using it, and it just approaching it has been enough to get people to back down. In the long run, it teaches them not to ask you about those things, which is what you want. I can see it being rather harsh, though. I guess I have some difficulty imagining being in an interpersonal relationship in which I both feel strongly positively towards a person (enough to make me reluctant to say something like that) and at the same time having things that I have to lie about.