I was curious about this myself because I've only seen it in pictures/videos. This car gadget offers pleasure more to the passengers than the driver itself.
They vary in shapes and sizes. If you choose the 5 or the 10 inch LCD headrest monitor I'm sure you will enjoy the ride.
Looking on different websites i can also tell you that they fit most car brands and models, they have built-in DVD players, the sound is transmitted infrared to wireless headphones and some even swivel for improving the viewing angle, making movie watching a thrilling experience.
I guess it's good for kids too, they are always full of energy on long rides, maybe some cartoons can quiet them a little.