I have a dual DVD system in my van. They can play either independently, or simultaneously. I got them for about 200 at Wal Mart. I bought the 2 yr warranty for an extra 20 dollars (I think it was 20).
Overall, I like them. A few times the DVD has frozen up. I am not sure if that is the fact my DVDs are a bit old, but it does not happen that often.
I drive 500 miles a week. About half of that with my kids, and the DVD players make it a lot quieter. My kids are 6 and 9, and both autistic.
Although they have these players, sometimes they do act out, but that is my children being just that, children.
I have heard that ceiling mounts can be a pain in the neck to have serviced, which is why I opted for these, they tie via strong velcro on the headrests of the front driver's seat and front passenger. My van has 3 rows of seats, if that helps.
Another thing about my DVD players, not only do they plug into the cigarette lighter, so if you have one of those type outlets other than up front (some vehicles have them in the back of the middle console, and some vans have them in the back of the van). They have an adapter that you can plug them into a regular wall outlet in the home.
In my personal opinion, an intergrated DVD (or a pair of them) into the ceiling of your vehicle, can be a pain to service if needed, but more out of the way than the tie ons, but the tie ons are more portable, maybe more cost effective (I don't know what intergrated ones go for), and frankly, I just like the idea of being able to take them with us into the house if the kids want to watch something in their room, or if you want to take them into a room with no tv.
I hope that helps.