When i was installing a few years back it would really depend on the persons budget. Audiovox made a really nice unit back then that was fabric/color match of various factory head rests.
i have also seen where people got units like this and had them re-covered to match. a auto upholsterer should be able to re-do these for about 100 each headrest. keep in mind these are not the same shape as the factory headrest. where as the ones i talked about above were.
Audiovox AVXMTGHR1D 7" Headrest Monitor w/ Built-In DVD Player
The other option is to get a "*****" monitor and physically cut the factory headrest and place it in there. For people that didnt want the expensive factory kits or wanted larger screens than the one i listed above this was an option.
I am also considering doing this in my 2015 but i am looking probably at doing the non-dvd versions as i can just stream from my phone the video signal and do a netflix or sling tv.