I have a rooted Samsung Infuse and Spotify. I pay for the premium version so I can enjoy streaming Spotify.
When I first tried to stream a song, it would play the entire song except for the last 5 seconds of it. I would manually need to select the next track for it to play. The next song would do the same thing, stop the music for the last 5 seconds of the track.
I figured it was the 3G/4G being cut due to JuiceDefender. But I have heard from others that they can use Juice Defender and Spotify without the track being cut.
You need to configure JuiceDefender to play with the screen off. That is what happened to mine, too. I don't think you can customize JD with the free version, though. I'm not sure. I have the Juice Defender Plus and have added Spotify, Pandora and other sound apps to play while the screen is off.