I've only worn my officer on a couple troops so far. All I've gotten with that is "what military do you represent? Oh! Star Wars! Right!"
But in my Guard costume, I usually just get "Red Guy" or "Vader." I'm always flabbergasted by being called Vader. Have these people paid any attention to pop culture? I did have one guy in his 40's run up to me once, though, and say "Imperial Guard! You were my favorite growing up!" and took a selfie with me.