Baby basket

The baby basket is fastened to the wall of the aircraft or placed on the floor in the legroom during the flight. The child can sleep and lie down in the basket during the flight with the cover zipped up. During take-off, landing and turbulence, the infant sits on the lap of an adult with a separate safety belt fastened. The adult is allowed to have only one infant sitting on his/her lap. The maximum inner length of the baby basket is 70 cm and the infant using the basket may not weigh more than 11 kg or 24 lbs.

Have you ever used one? Where are they on the plane and how does that actually work? The airline doesn't mention extra charge; did you have to pay for it though?
thanks