As the service provides very good quality, legal, audio streaming with rather massive music selection, it has already become extremely popular in the countries where it is available (Spotify is currently available in the UK, Finland, Sweden, Norway, France, Italy and Spain) -- partially cutting down the interest towards illegal music downloads. Our Finnish users have been praising the service for couple of months now and most of them seem to be extremely happy with the audio quality as well as with the usability of the product. Spotify has also announced that they're developing client apps for various mobile platforms, expanding the service beyond PC desktop.
Couple of screenshots from our recent -- Finnish language -- review of Spotify, giving you an idea of the client app GUI:
Fig 1. Searching for music with Spotify
Fig 2. Lazy man's option -- use "radio" to just randomly listen specific genre or decade.
Fig 3. Building playlists..