It looks terrible... A beautiful natural belly button is still the best! Especially the holes they have to make look disgusting when you don't wear it - and it's a real turn off: can't even play around the belly without worrying to pull it too hard by accident.
If you want some decoration I'd suggest to use those little gems you can just put in your belly button, there are some "glues" that hold it in place but won't do any harm to your skin there. It looks great with it and still no holes or other injuries when you take it off!
I think belly piercings are extremely sexy on a well toned stomach. They should be tasteful and cute, though. I've seen some really ugly stomach piercings and think to myself "Why did you even get it pierced if you're just going to stick an ugly bar through it?"