Amazon Cloud Player (free) for iOS gives you an option of getting all your music from the iTunes App Store, and its simple interface is easy to pick up and use immediately.
Whether you have music already from Amazon or not, you can quickly log in with your Amazon account and listen to tracks already on your iPhone. If you have downloaded music through Amazon, with the app you'll be able to listen to both Amazon- and iTunes-bought music.
Once you have music uploaded to Amazon's Cloud drive, the interface is dead simple. Two tabs let you switch between Amazon or music on your device. On the bottom of the screen, you get standard playback controls. When viewing your music libraries, you have buttons across the top so you can sort by playlists, artists, albums, songs, or genres. You also can create new playlists from within the app directly. When a song is playing, the app switches to album cover view and adds buttons to the playback controls so you can shuffle or loop songs. There's nothing new here, but the controls make it easy to get to your music wherever it is.
One thing you can't do on the Amazon Cloud Player is purchase music from Amazon. You'll need to go to the Amazon Web site on your computer to purchase songs and put them on your Cloud Drive first before you can stream that content. Even if the app is currently open on your iPhone, you only need to hit a refresh button to add recently purchased songs and start streaming.
One thing I found in the help files is particularly important for those with limited data plans. In the settings, Amazon suggests you make the selection for "Download and stream only on Wi-Fi." With this option turned on, you won't have to worry about overages on your limited data plan, but you'll obviously lose the capability to stream music while on the go.
The killer feature with Amazon Cloud Player is that you can upload up to 5GB of music (even from your iTunes Library) to Amazon Cloud Drive for free, then simply stream the music so you don't have to have all those audio files taking up space on your iPhone. Amazon Cloud Drive also offers larger storage plans for those who want to add a huge library of music, but you'll need to pay an annual fee for the privilege.
While there's nothing groundbreaking about finally bringing the Amazon Cloud Player to iPhone, people who want to take advantage of the streaming capabilities or want to play music from both their iTunes library and Amazon should definitely download this free app.