Since I'm new.....I'll answer this.
No. I never have. I also have a different opinion about this than most probably do. I think that even if I had my knife on my at a time where there was trouble I would probably not use it. I would first try talking and then fighting with my hands. If the person had a gun, I would just give up my wallet or whatever I had they wanted. If the person had a knife I may do the same thing. If the person had nothing I would simply fist fight them after trying to talk my way out of it.
I would just be afraid that my own knife would be used against me if I lost control of it.
I think I would only use it in an absolute last resort situation. I know of guys who pull their knife out if a fight comes about and they do it early on. I just don't do that nor do I think it's wise.
Just my $.02