I would use spotify if it made sense to use it on an Iphone. The interface is horrible. Still dont know how to turn shuffle off. K.I.S.S. keep it simple stupid. I use other apps because trying to use this app sucks
If you are subscribed to Spotify Premium, you are able to listen to any track available on Spotify's catalog on-demand. If you're currently not a subscriber, then you will only be able to listen to playlists/albums/artists on shuffle.
With all that said and done, I would have to argue that Spotify (on iPhone) couldn't possibly get any more simpler.