There´s one very important thing I forgot: Taxi-drivers are usually very friendly and open to all sorts of chatting during the trip. They often know things not even a Madrilenio (cago en la leche, mira que no tener "enie") knows, cause they talk to so many diferent people everyday.
Regarding the airport thing I forgot to mention, that more than once I´ve heard drivers say all kinds of "not very friendly" things about their colleagues who rip off tourists.
Hasta la próxima!