Frankly when I see them I think a girl is trying to draw your attention to her stomach. I associate them with sluts kind of. They can look nice and sexy if you are doing it specifically for your guy but if doing it in general it gives the impression that you are trying to look sexy and draw attention to yourself.
Pretty gross in my opinion. Not a fan of piercings (excluding ears) they just seem unnatural. Of course everyone has a different opinion and there are a lot of people out there that would think it looks great