I have and do. The knife is interesting. Easy to use and hard to defend against. If the other person is trained in its use and you are not.... well you are in deep deep trouble then. Against a trained knifer going to guns at close range is not even a option unless you really know what you are doing.
Not trying to rain on any parades, but I feel it is a overlooked aspect of personal defense. I mean most do not have a plan or skill set for a close range knife attack. But a knife can be carried almost anywhere, almost anytime.
I don't expect to change any minds, knife work is a "If I don't think about it I don't have to worry" type of thing.
I was involved in a situation where a knife in my hand and a stare down caused a thug with a gun in his waistband to rethink robbing me.
I think he knows, that had his brother not been holding a gun to the restaurant managers head he would have been carried out in several bags.