I usually get, "Nice plants, but where are the fish." quickly followed by, "Oh wow!!! They're gorgeous! I've never seen these before." Never underestimate the power of Darios!
I'll occasionally get, "The tank is so tiny, but you made it look so much bigger."
I know I've really impressed them with my nano tanks when someone says, "Where can I find this stuff?" So far, at least 3 of my friends have set up their own tanks after seeing mine.