Planetary starts you out in a galaxy with stars representing the artists in your iPad's music library. Zooming out from this view will give you an alphabet wheel that will allow you to navigate to artists sharing the same initial letter. I'm sure it was technically possible to show every artist at once in one giant view, but this probably would have been an impractical way to navigate through a huge library.
Somewhat counterintuitively, you can't zoom in on a single star/artist down to its system of planets/albums by using the two-finger zoom gesture. Instead, tapping on the star or its name will zoom into the system view, showing all albums by that artist as orbiting planets. These planets are skinned with the album art, which makes each planet visually distinctive.
Zooming in on a planet/album will reveal its system of moons/songs. Moons are sized relative to the play counts of the songs they represent, and they orbit at a speed proportional to the song length. If you choose, you can even enable controls that show the path each moon sweeps out as its song plays, which provides an extremely imaginative twist on a playback indicator.
The downside to the interface is that a lot of the functionality users take for granted from the iPod app isn't present in Planetary; there's no way to navigate playlists or shuffle songs, for instance. The good news is you're not completely dependent on Planetary's interface. You can switch over to the iPod app to navigate through your library and set up the music you want to play, then switch back to Planetary and watch it do its thing.
Music visualizers aren't necessarily the first thing that comes to mind when people think of apps they need or even want, but as soon as I saw the writeup of Planetary on MacStories I knew I had to at least try it out. I haven't been disappointed; in fact, Planetary is one of the most visually arresting and intriguing apps I've seen on the iPad so far, and it's an app that literally had me saying "Wow," out loud within moments of first using it. Highly recommended.