I've never bought anything but alcohol at Duty-Free shops- the rest of the goods, other than cigarettes, don't seem to be bargains. It's just another way to get tourists to spend money while they're stuck waiting for flights. If I want to shop, I'll go to the mall.

We had the same unpleasant surprise at Rio Duty-Free that someone else mentioned- US Dollars and credit cards only. I've heard that's also true in India.