I am officially impressed. I bought a Fire TV 4K as I signed up for Amazon Prime just for the free next day delivery & subsequently got the Prime Video & Music for free. I had been using a Sony Blu-ray player to access Prime Video but recently this has got very slow connecting to the network so I bought a Fire TV HD to use instead & have been really impressed especially when I found that I could side load Kodi & access my media collection that previously I had access with a Boxee Box. I then thought that I would try Plex & was even more impressed with how slick it is.
I had been setting up a Seagate Personal Cloud 4TB NAS just to serve media to Kodi on the Fire TV but discovered that it can be set up as a Plex server & apart from some DVDs that had been ripped to Video_TS folders the Fire TV has played everything without a glitch. I was initially concerned that the NAS wouldn't be powerful enough but it performs fine although as a NAS it's disk read & write speed is slow. I't's become a little addictive tweaking filenames & selecting alternative posters & backgrounds to get everything "just so". This combination of Fire TV 4K & Seagate Personal Cloud has proved to work very well. Kodi is more flexible but Plex is much slicker. The Boxee Box always used to garble the first second or 2 of video & audio but Plex plays really smoothly.
I see people praising the Nvidia Shield as an Android TV player but it's nearly double the price of the Fire TV & as I have less than zero interest in playing games isn't appealing. The Fire TV 4K looks like great value for money as not only is it a great Plex client but has loads of other facilities like running Kodi really well plus other apps like BBC iPlayer & Spotify plus of course Amazon Prime Video.