I used it on an electronic form where it was an option. I have a bit of an issue with electronic forms that don't alow you the option of no title, which is my preference, and like having a choice of random titles which aren't just "Mr, Mrs, Miss, Ms." I hadn't seen Mx before, so it had some novelty value.
Anyway, then had to deal with a real customer service representative, face-to-face. He was totally confused by Mx and thought it was a computer error or something. I decided not to add to his confusion or education and just said if I had no choice about having to use a title, then I supposed Ms would have to do.
Whatever one thinks about titles like Mx, I do think that if they're being offered as an option on an organisation's systems, then that organisation has a responsibility to educate its staff on what titles might mean and who might use them. If they actually need to know someone's sex and gender (and the NHS is one of the few organisation's where it is relevant, ) that should be recorded in a separate field. After all, Dr Brown or Rev Smith could be trans and not want to use Mx.
I am somewhere between mix and mux on the pronunciation front, with a preference for mux, if I have to.