This entry from Jonas Andersson's liquidculturblog has called my attention as I have just tried out spotify myself. I only tested it briefly but wondered what it can do what last.fm cant already do better together with many other things. But here you can read a much more informed opinion.

Differing attitudes towards Spotify - The discourses around the music streaming service Spotify show how Internet users are split into two rather different groupings: those embracing Spotify, and those who do prefer wilder, perhaps more interesting services like Songza and SeeqPod. [The Liquidculture Notebook]