Visually, it's impossible to distinguish between songs and albums because all you see is an album cover, and must tap it to show whether you added a whole album or just one track. Also, albums that I revisited a second time were confusingly listed multiple times under Listen Now.
My Library shows the genres, artists, albums, and songs in your library (either local or everywhere). From there you can drill down into albums to play (although art was often missing from places it was supposed to be), or easily start a radio station based on a song, album, or artist. Tapping an artist name takes you to that artist's page on All Access and shows you the artist's music that's already in your library, as well as top songs, top albums, and related artists. During my time with the service, the related artists seemed pretty accurate.
Playlists shows your recently created playlists, auto-generated playlists based on what's in your library, and then all your playlists at the bottom. Adding an album to a new playlist unfortunately doesn't name the playlist after the album. Instead, you get a generic Playlist 1 with the opportunity to rename it.
Radio shows you any stations you've built, and also gives you the option to create a new station. When you tap Create new station, you're prompted to search for something to base it on. On the S4's screen, the search box reads Search for an artist, so.... I assume this means you can also search for a song (and perhaps other things) to base your station on, but the truncated label makes it difficult to know for sure until you actually begin. The results come up as you type (a nice touch), and offer you artist, album, and song stations to choose from.
Finally, Explore lets you look around the content by genre (and sub-genre); offers recommendations; displays featured playlists and top songs/albums; and shows you new releases.
At almost any point in your travels through the app, you swipe from the far left of the screen to the right to bring up and access the five master navigation elements. (I say "almost" because when in a full-screen song view, such an attempt is met a glowing blue light that essentially says, "I know what you're trying to do, pal, but no dice.")
Even though the app works in landscape mode, the UI is clearly not designed for it. For example, the image at the top of the screen on an artist page zooms nicely enough when I rotate the S4, but the interface doesn't do much to take advantage of the wider space. You just end up with less information on the screen.
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