Spotify has really come into its own recently and that’s because of its accessibility from elsewhere.
The ability to run Spotify as a background task on the iPhone has finally made it usable on the device. Until then, it restricted doing any other task until music had finished playing. Now with background operation and integration with the iPod controls it finally offers a great experience. Also, with a 16gb phone I can download a ton of tracks for offline listening.
ShareMyPlaylists.com is a great service that provides user generated content of playlists for Spotify. It provides a massive ecosystem for discovering and listening to music. Their iPhone app also works with the Spotify app.
Spotify has still not broken through into the mainstream yet. It still seems to be a tech audience which gets it. Perhaps the success of the iPhone 4 willl lead to success for Spotify too.