Duty free can often be the an opportunity to purchase items for a nice discount by waiving the obvious taxes on items. Often cigarettes and alcohol receive the highest discount due to the normal duty placed on such products. Some of the biggest bargains are UK airports. However discounts are scarce on on the likes of watches and perfume.
Sometimes it might be necessary to pay duty on some products bought duty free, if you have exceeded a limit placed on your home country. For example in the US, you have a limit of $800. So if you go over that, you may end up paying some duty at your port of entry.
Certainly the advice would be to no go mad at duty free and being some items back, but do you homework if you want to bring a lot back. Remember you'll have that extra weight to carry around too.