Same thing happened to me yesterday - honda shadow pulled up beside me and said nice bike, I replied with a thanks and road off.
My normal reply is always Thanks
Now, if they say "nice sportster" then the response is entirely different. LOL
HA.
Surprisingly, nobody has mistaken it yet. People around here seem to know their bikes.
That's funny, because when I had my sportster, I never had anyone say nice sportster, it was always "nice bike, is that a fatboy?, or hey, nice night train. Is that one of those V-rods?"
When I got my dyna, all I started hearing was "nice sportster", and "cool bike, is that a sportster?"
When people compliment my bike, I just say "thanks", no matter what they think my bike is, lol.