I have been reading up on Plex on the Roku and from what I gather there is basically one view for movies, tv shows etc. Is it side scrolling thumbnails of your media? I cannot see myself scrolling through over 250 movies to find what I want to watch and certainly my wife and son won't do that either. Plex looks rather bland compared to the Apple TV and certainly the Mac client interface but thats to be expected. Is it really that neutered?
I'd say the eye candy of the Roku is pretty good. Plex is similar to Netflix in that there is a menu item for recent additions. That lets me quickly find my most likely viewing choice. I have a large video library of everything I have purchased on DVD and Bluray over the years.
You can create your own "groups" for things like movies, so I have one for "Classics", "Home Video", "Kids Movies", "and "Recorded TV Shows".
Plex automatically grabs all movie metadata, allows me to change the cover, etc. It transcodes as well, so I store movies in .mkv format or whatever, and they simply play.
For me PLEX is also appealing because there is an iPhone app, iPad app, and even one for my Samsung TV (so for that TV I don't even need a Roku). PLEX allows remote access so I can play my movies or Recorded TV shows on my iPad from anywhere in the world.
I also snag (one click shortcut) web video's with "Plex It" and these are instantly linked to a PLEX menu item called "queue". Plex also has a channel for YouTube which gets around the fact that the ROKU doesn't have a native Youtube app. There are lots of channels available using PLEX.
If Apple ever opened up the ATV to an App market, I'm sure PLEX would be available, and then I would dust off those devices and use them again. However, the Roku's would remain as well.