The music is organised in two main hierarchies: artist/album/track and genre/artist/album/track. There's also a "new music" hierarchy that goes date/album/track. (The first is hopefully useful for people trying to find music by name, the second for new users exploring what's available, and the third for long-time users who just need to occasionally sample the newer stuff).
Both hierarchies are available on the Squeezebox's display and also in the plugin's web pages. The web pages have "more info" and "buy" buttons that go to the appropriate pages on magnatune.com.
If you have a bit of time to play with it, SlimServer comes with a Squeezebox emulator (SoftSqueeze) which should demo the plugin nicely.
I asked Ian to send me some screen pictures, which he has, and I'm displaying below. Very, very nicely done!