I have had this happen and I felt guilty because I didn't find him physically attractive. Mentally I was attracted to him but not physically. I could tell he wanted to know if I found him to be attractive and I had a hard time inside about it. I wanted to find him physically attracted but even after a few months of talking I didn't. We planned to meet...he was a great guy and I want a great guy but he ended up getting caught in some very shady stuff he couldn't deny and so we never continued...problem solved. :(
I mean if that happened I would really just say aw well thank you and keep it moving. Would I be flattered? No not really I would appreciate it though. Would I be creeped out no not really unless he was being creepy in the way he said it lol like "Oh damn baby you look pretty today" rofl that's not okay that would get him kicked in the shin
I would be quite flattered and happy but I'd also wouldn't want to give a stranger or my "friend" the wrong impression. Generally a girl would "deep inside" be smiling big but we'd also wouldn't want to give the wrong impression if we had no feelings for the person who said it. If it was by a friend and I knew he didn't mean no harm in saying it, I wouldn't strug him off I'd thank him.