I know I may be sounding like an old fogey - but what's wrong with just looking out of the window and/or chatting?
If everyone starts using these contraptions, will it mark an end to the classic 'are we nearly there yet?' line? Surely kids are meant to be fed up on car journeys?
(BTW I know someone who uses one regularly for journeys of approx 30 minutes cos otherwise her DCs get bored!).