If there was ever an app that worked exactly how it was intended, the eMusic app is it. Every bit of the eMusic service is available in this app. You can buy, download and stream music, you can browse their entire library and sample songs, you can listen to special eMusic playlists, and you can even sign-up for the service and manage your account, all without ever entering a web browser.
If you're not familiar with eMusic, basically it's a subscription service for music. You pay a certain amount per month, and you get to download that amount’s worth of music. The key here is that most of the songs available on the service are cheaper than on iTunes or Amazon, but the trade off is that those services have a much larger selection of popular music than eMusic does. You'd want to use eMusic for the indie stuff, essentially.
There is value in this service, as most of the songs available in the eMusic library come at $0.49 rather than $0.99. So you will save money as long as you can find what you're looking for there. On the flip side, you can't buy entire albums at once; you have to pay for each track individually. So it's best to weigh your options carefully before buying from eMusic.
But, if you're into this service or already a subscriber, this app has everything you need to get going. Really, everything you can do at eMusic.com you can do here as well. It's a fully comprehensive app, and very useful.