I been using Spotify since its US launch. They have a great library. The guy who started Napster is with Spotify. Which is hilarious, because he and Metallica just had a news conference on stage together announcing Metallica's catalog being added to Spotify.
I will say this. If you spend at least $120 a year in music, then you might as well be a premium member.
The streaming is good, but can be subject to weather and signal reception. Same goes for Rhapsody as well.
If you want absolutely zero interference. Then Spotify allows you to have 3,333 songs for offline listening. So basically I keep my favorite lists available for offline and a "starred" playlist which I update regularly.
Spotify has also been improving their radio station feature as well. I use it for when I want a little variety from my playlists. Works the same as Pandora. The station will play songs based off an artist or genre.
My wife got jealous and started an account for her as well after she got fed up with iTunes.