I'm not sure how prevalent it is, but I know some carriers like United[1] and Delta[2] (via Gogo) stream media to your personal device over the aircraft's WiFi.
> Make sure their device is charged?
Many flights now have USB ports and/or power outlets under the seats.
[1]: https://www.united.com/web/en-US/content/travel/inflight/ent...
[2]: https://www.delta.com/content/www/en_US/traveling-with-us/on...
The USB ports are nice; especially since I have a syncstop so don't have to trust them. The video, however, is only available through their United App (Not even the Gogo Video app, available on a few other carriers, but the United one is required to play videos on their flights). Okay, so I went to download it. You can't on the flight - They block internet unless you pay for it ($15 for my two-hour flight! Though I could have gotten a half hour for $10), but I had a layover, so I downloaded it in the terminal. I was taken aback - It wanted access to my contacts, my phone information, my text messages - Well, thankfully Google has profiles on tablets, so I loaded up my burner profile.
But actually on the flight, the experience was horrible. The video constantly stopped to buffer. There were no media controls - You pressed play, and you could pause the video by tapping the screen, but you couldn't scrub back or forward or restart the video. The selection was okay - I watched some Lego Freemaker Adventures (I'm a sucker for the lego animations, and Star Wars, how could I resist?) but there were a variety of movies that I'd have watched instead. I gave up, though, after my second layover I went to boot up the episode I'd been halfway through on my previous flight, and it started over. I don't blame it for starting over... but the lack of media controls made that a non-starter.
Ultimately, streaming to personal devices (with rental tablets available) is going to be the clear winner; The expense of maintaining those headrest units has got to be significantly more than the little media server on in-flight wifi, and the complaints I have about privacy invasion are yet another sacrifice that airlines are all too willing to make on our behalf. But it's not there yet, and I sorta doubt it will be for a while yet.
> This free service gives you access to a library of movies and TV shows that you can watch on your personal laptop, Apple iOS device or Android device.
The wifi is decent - I was able to stream 480p video without interruptions, which is way better than I expected.
Seatback systems may not be great but they might be good enough to last a while longer.