I haven't seen this before, but at a quick glance, here's what I love about Plexamp that's missing for me:

* Similar artists, tracks, etc * Auto-playing similar after listening to an album * "sonically similar" tracks/artists - not just some arbitrary decision that some artist is in the same genre, but that the songs have similar sonic profiles * Artist/track radio based on all of the above * "library radio": smart shuffling * artist mix builder: creates playlists based on some artists that you choose (and includes tracks from similar artists automatically)

I have too much music to know what I want to listen to all of the time and I don't want to sit there carefully curating playlists and trying to discover things: Basically, I want a system that's smarter than I am to tell me what to listen to (but with my own music that I own, obviously)