There are items that are definetely worth buying in duty free, but there are also a lot of items that you can get for the same price or maybe cheaper online and are not worth the hassle. Items like chocolate and alcohol are best to avoid, however high fashion and cometics are very favorable to buy.
Let's for example take the Gucci Dionysus bag, this is a bag that I have been eyeing for ages. However It's most likely that I would buy thig bag in the duty free zone rather than in the store. A Dionysus bag like the blooms print shoulder bag has a store price of $1960. The same bag, but sold in the duty free zone of London Heathrow has a pricetag of $1794. This is a major discount, and this bag is so much in favor that it probably will never be reduced or available in an outlet.
Therefore when really having your mind set on a designer piece, do some research before and maybe you can pick it up on your way to paradise or let a family member pick it up for you