I've been running Plex on a HP Microserver (Gen 2, IIRC) for a few years now, in conjunction with a couple of Roku 3's. For the most part I've been happy but it's starting to show it's age. I also have two Chromecasts but pretty much do Netflix/Youtube on those.
However, I recently picked up a Chromebox and threw Openelec on it. Why? Because although I love how transparently Plex manages my media libraries, transcoding was absolutely killing my server. That, and the fact that the Roku doesn't support local decoding/mixdown for DTS/Dolby, doesn't support DTS-MA or TrueHD, only supports RGB, and cannot do 10bit decoding, left me wanting more. I've been ripping my Bluray collection to my server/NAS and have been going with higher quality encodes, but using the Roku as a playback device results in a washed out, lower quality experience everywhere but my home theater (where I can direct play).
In contrast, XBMC has played every single file I've thrown at it without a hiccup, including the advanced formats, and even interfaces with my Plex library via PlexBMC.
This is pretty close to the ideal configuration for me, as I can have anything from a dumb TV to my full blown home theater connected to the Chromebox, and every file in my library can be played back without any transcoding on my server.
That said, I'd love to see a Roku 4 with more features and/or some other alternatives come up - the ease of getting the Plex/Roku combination online was a pretty killer point, whereas XBMC has required a little more tinkering (albeit with much greater flexibility).