I've installed Plex on my new Apple TV and so far I'm happy.
As with all new programs Plex does require some improvements, but after using it several days by now I can simply state the benefits and downsides of using Plex on the new Apple TV.
Plus sides:
+ Standalone now so your iPhone / iPad can be used for other things
with plex running on your Apple TV you don't need to use Airplay any longer, this is a big plus since using Airplay with your iPad or iPhone could result in breaking of the connection once you start transporting the signal using Airplay and start browsing at the same time on your iPad or iPhone. For example, when visiting a website that has an mediafile starting to olay automatically would result is breaking of the Airplay, or rather, it would start playing that mediafile of that website instead of continuous playing the moving you're watching.
It's not a deal breaker but I noticed that while watching movies using Airplay I also used the iPad / iPhoen for other purposes sometimes with sometimes bad results for using Airplay.
+ Visually pleasing
I love the fact that every appplication on the new Appe TV has more or les the same 'light' touch concerning look & feel. Browsing through your serie episodes is a joy and fast. It also remembers which episodes you have been watching alltough this requires a reload of the page section. In short, Plex looks great.
- SRT files does not always work
It's not something that can't be fixed, but frustrating none the less. Where all of my srt files works on my iPhone and iPad they don't all work on Plex for Apple TV. Most of them, fortunately, works fine. But this is a serious issue that needs to be addressed.
- Still depending on a server externally
Like the Apple TV foto application Plex also reauires to download the content from an external Plex Server. I'm using my Mac Mini for it that is stored in the attic where I'm working from time to time. A big miss with the current Apple TV is the ability to attach an external drive to it so Plex could access those media files directly. Although my network is fast it does require some time before Plex can fire up the movie or tv episode of choosing. But this is, for me, not a deal breaker because once an episode or movie starts playing it's very stabile, more when compared to the iPad or iPhone.
This aspect is way more disturbing using the photo app on Apple TV, it requires to constanly buffer the pictures from your iCloud and it doesn't store the pictures on your Apple TV.
So both plus and minuses. Overal I experience Plex to be very stable but it does requires some updates but so does Apple TV. Especially the ability to store photo's on your Apple TV from you iCloud permanently and a much much much faster Apple Music. I've got a 211 mbit connection so there should be no excuses why I need to wait 15 secondes, sometimes even longer, before a song is been played. Again same story there, an option to download all of your playlist on your Apple TV.