The key thing to know about duty free is that it is no bargain, especially if you live in the US where retail taxes on liquor are generally quite low. It is, however, a great way for politically connected folks to get a very profitable franchise at the airport preying on the unknowing.

I can buy booze at my local Trader Joe's or Costco for much less than the duty free prices.

Years ago you could get quite a deal at the airport. Now, IMHO, not even worth thinking about.

If you live in a higher tax area such as UK or Norway-Sweden there may be some savings.

In my experience local booze is quite cheap in the Islands. Just buy it there.