Santa brought one of these about three years ago, and we used it quite a bit. Our car doesn't have a built-in DVD player, but this unit fits perfectly between the two front seats so the kids can both watch from the back seat. Other vacation benefits: you can take it into the hotel room, and it also plays music CDs. My husband is the type who loves his kids, but needs some separation time every day; he loves to take the DVD player out on the balcony and watch a grown-up movie.
Sadly, our original player bit the dust. We bought a second one (Polaroid brand) and it isn't quite as nice as the original one -- it seems a bit more "flimsy". They have come down in price since we bought the original one. Both of ours have been able to play two movies before needing a re-charge, and they've both come with power cords for the car and the house. Both of ours have had two separate headphone jacks, which is fine for our two children. If you have more, you can buy a splitter at RadioShack, so you could potentially split your sound four ways.