i realize i'm chiming in here a bit late, but we just got back from a 3-week car trip journey, and we brought ours.
this is the one we have:
http://www.amazon.com/Kawasaki-Mobile-Players-Monitors-PVS297OS/dp/B0007X1KX6we bought it a couple years ago (which is why it's out of stock now) when my ILs moved 12 hours away. and we plan on using it for a LONG time. both monitors can be plugged into the same movie (which is what we do now), but when they get older we can plug the monitors into 2 separate movies & they can then wear headphones (which we'll do when they're older)
and yes, since i do have friends who seem to use it even for 5-minute trips, we laid some ground rules with each other before we let the kids see it:
* we only use it on trips that are 3+ hours (so not used for grandma's house 2.5 hrs away, but used for nana's 12 hrs away)
* it's only used after lunch - this is how we made peace with ourselves about watching tv in the car.
like deb said (& everyone echoed), we saw it as just another tool. by limiting ourselves to after lunch, we were also trying to time it so that by the time the movie was over, we were at our destination (and yes, there are some days where we just watch one after another because of lousy traffic. but those are few & far between.)
after lunch is also when the kids were crankier from being in the car - in the morning when they were fresh, they were content to chat, color, read, and do whatever. i had a box of books/toys inbetween their carseats, and they would rummage through until they found something they liked.
