Spotify is the first streaming service that I've consistently used. I've tried the others but none of them "stuck" with me. The unique thing it provides is full access to albums played any way I choose.
I use it to listen to albums I already like and to discover new artists. When I'm looking for new artists, I don't want to hear one or two songs ala Pandora. I want to listen to entire albums. I usually need to listen to songs two or more times before I can decide if they're really great. Spotify lets me do that. I can queue up (via playlists) artists that I've discovered now but don't have time to listen to until later.
It's changed music's place in my life, seriously! Before Spotify I was stuck listening to the same old stuff from my collection. I thought I didn't have enough time to find new artists. With Spotify, I'm listening to new stuff all the time with little effort on my part.
When I want to listen to a mix of artists I listen to a professional DJ mix or a decent radio station. The Pandora model just doesn't do it for me.
That said, it REALLY sucks that Spotify is in bed with Facebook. I was contemplating the paid premium Spotify membership but no way in hell will I associate my billing information with a fucking Facebook profile. Fuck that!