Some time ago I wrote the short article regarding RPi and mpd. That post is a small update to mentioned article.
I realized, that it would be nice to have also spotify on my Raspberry Pi.
To be able to use spotify with unofficial client you need to buy spotify premium account. IMO it's price is not from the cheapest, but acceptable. For the solution that I am writing about it's not really needed, but useful if you hate advertisings ;-).
First I found, that there is a solution called pimusicbox. It is based on Mopidy, but everything is preconfigured. Devs idea is to gather many possible ways to use Raspberry Pi as music server in one solution. It should be good for not technical users, that are not able to make config on their own. It uses web server and all possible options could be set there. In pimusicbox you can search for spotify songs or artists using that web interface. If you will enable mopidy spotify web plugin you could access your own playlists and few more options. As for me it wasn't a good solution, because I didn't want to be forced to use web interface for the whole time. Theoretically you could access some of the settings using your mpd client, but it wasn't fully working for me.
Later on I was more lucky and found better solution that is called raspotify which makes your RPi available as device for Spotify Connect. It is really useful, because you are able to access all songs, artists, playlists, radios etc using officiall app on mobile phone. I like it, because I am able to switch quickly from my RPi in the kitchen to second one in living room and continue listening the song from the same moment :-).
Installation is really easy, because you only need to add repository and gpg key and you may install or update it using apt-get.
You may edit its config
/etc/default/raspotify, but it is not needed.
You can find my changes below:
PS. If sound is too low you need to check your alsamixer settings :)