The app seems to have the potential to be useful to those earnestly seeking engagement rings, and those looking make trouble for the soon-to-be betrothed.
I'd divide these categories roughly by gender, and say that this app could be the most useful for the menfolk.
He, who is most likely uneducated on the subjects of the on-hand impact of one carat versus two, not to mention shape, setting, metal or design, will be able to see rings in their actual size. (With price range.) Also, there's the "ring sizer" -- he only needs to snag a ring from the beloved for a few moments, place it on the iPhone screen, and line it up with circles corresponding to ring size. He can also make an appointment for a consultation right through the app.