I read this article in the Sunday Times this weekend. It's about the best duty free buys in various countries- perfume barely gets a mention, which is madness when the DF perfumes are one of my favourite bits of going on holiday!
I am recently back from Barcelona and so was lucky enough to go through the excellent airport there again. I often fly from Heathrow which it has to be said has an excellent scent selection including some unusual sniffs like the Hermes essence range. So it's tricky for lots of other airports (not cities airports) that I visit to match up. However what they might like in square footage they more than make up for with unheard of scents and lines that are no longer carried at home.
What I always long to find in the DF is an unusual Guerlain like Nahema or Lancome's Mille Et Une Rose. I haven't been lucky anywhere yet...
However it isn't really the buying that is the main joy of DF sniffing. It's the fact that you're in transit and there is very little to do so no one can really mind you spending a lot of time sampling and enjoying the scents.
I also think it can be very nice to pick a scent either at the start or the end of the holiday. If you buy at the start of the trip you can wear the scent throughout the holiday and it will always remind you of that place- and if you buy a perfume at the end of the trip it's like a last gasp of the holiday and a little pick me up to carry you through the drudgery of returning to work and holiday washing!