I am using Pandora in PC either through Boxee or Web Browser, so I can’t comment about using it on Iphone, but why I like and why I think its increasing its users base is because
– Its free
– I like the way the UI is designed, its simple yet captures all the needs like search, feedback like, don’t like etc
– Very good at creating stations which I think are like playlists which you don’t have to create
– Allows users to be exposed and explore the world music, because before using Pandora I was very rare listener of music , but after being introduced to it, I have started listening to music atleast 3 times a week.