Not good for the retailer
The problem I have with this kind of application is you are using the facilities of the shop to browse and look at items then seeing if you can buy it cheaper elsewhere. Not just from another shop around the corner, as you would do in Tottenham Court Road for those Londoners, but from companies that have a different business model who don't need to pay for the expensive high street front and the staff to help you.
If you want to use their 'value-added' services then at least have the decency to help them continue to provide that service by spending your money with them.
(It does look like quite a nice app though, I just have moral feelings about it's use)