This is some interesting data shared by Asymco that shows we are downloading apps much faster than we download music on iTunes. As a point of comparison, it took the App Store 2.2 years to be where iTunes was after 5 years. In other words, the App Store is hugely gigantic.
More interesting is that Asymco foresees App Store downloads will pass iTunes downloads by the end of the year. So why is the App Store so popular?
I think there is one main reason behind this: many apps are free. While you can’t get a song on iTunes for free, you can get thousands of applications for $0. If you were reticent about downloading a $0.99 song, there is no financial barrier between you and a free app.