This is what makes it interesting:
Mirage uses the latest AI/music information retrieval techniques to compute a similarity model for each song. So before you can use Mirage to automatically compute playlists, your whole music collection has to be analyzed once.
After it is analyzed just drag and drop a song you like on the Playlist Generator item in Banshee and a playlist with similar sounding music will be computed.
Mirage is by far not perfect! Yet :-) - Imagine it is a radio station playing songs from your collection.