One of the biggest complaints about using Plex on Roku is how bad the interface is. In the past, they stuck with the Roku design guidelines and it lead to an interface of a bunch of bubbles. That is all over now as Plex has released a new channel (currently in preview form for PlexPass members). This new interface follows along with the design they've been using on game consoles and new Smart TVs.
If you are a PlexPass member, you can grab the new channel at the following URL: https://owner.roku.com/add/PlexPass