Yesterday I tried the Plex integration with the WiiM amp. On my iPad it works ok, I can search my Plex music collection and show an alphabetical list of artists, albums, etc. Music plays ok, but there is one big problem, the Plex server is not showing (in the management console) that it is playing music. So as a result, the tracks I played are not scrobbled to Last.FM which is what the Plex server does when I am playing music via Plexamp or the Plex app. So for people using Last.FM, this way of integrating with Plex is not a way to go.
On my Android phone, the Plex integration is garbage, cannot list items alphabetically or search my collection.