Are Car Dvd Players Good Or Bad For Kids?

I have 2 kids of 4 and 1 years old. I’m considering getting them a DVD player for the car to keep them entertained during long journeys.
I’m a bit worried that it could make them want the TV all of the time and the repurcussions if the DVD player is not there or we turn it off.
What do you think? Do you have good or bad experiences with this subject?