http://www.flyertalk.com/forum/showthread.php?t=826374
I bought a gift card for 100€ (which I can use to buy groceries) and got 2 return tickets within Spain on Air Europa (FB partner). I had to pay the taxes and fees (23€ to any Balearic and Canary island).
I got a Europe 'free' ticket on Spanair (Star Alliance) by spending 300€ on an Amex card. In Spain, there is no cash-back credit card to my knowledge. And 3% would have earned me 9€...
I tend to greatly prefer those offers rather than legacy carriers' offers and use the cheapest option otherwise.
I will use my incidentaly earned miles to expensive destination (think Africa).