Really depends on the shop, not many in my experience have the facility to take them. I bought quite a bit of pandora last year and paid on credit card, they "borrowed" the machine in the bigger jewellers next door. Also, you don't get the discounts you would perhaps normally get through a good bit of bartering, as it costs to process credit card payments.
Depends on your card and the rates they will charge you, but you might be better off getting a cash advance from your credit card in terms of getting more for your money, if you are likely to be a tough barterer!
