The letter below is in Spanish, but it basically says that Spanair agrees to pay 400€ due to a delay of nearly 4 hours in a flight from Madrid to Tenerife. This delay was very annoying, since they didn't even bother to offer us drinks or a decent meal. I was quite upset by this, so I decided to fight and found that the 400€ compensation is the responsibility of the airline, according to the European passenger rights (http://ec.europa.eu/youreurope/citizens/travel/passenger-rights/air/index_en.htm), but as mentioned in this article "[...] airlines are notorious for trying to wriggle out of their responsibilities and you may have to insist on your rights".
And indeed they try to wriggle... In my first formal complain to them, they just offered a one-leg national flight (not including taxes and administration charges) to be used in six months. This is a very poor form of compensation, since they are actually trying to make you buy another air-ticket with them, but with some discount. As an individual it would probably be a bit cumbersome to get beyond this, but being a member of the Consumer Association OCU (http://www.ocu.org/), I forwarded the case to them, who offered to act as an intermediary, and in no time I got the letter below.