Shawn Blanc writes a good overview of the current cloud-based music services. According to him, Amazon and Google provide a poor experience. Uploading your library does not seem to be the best solution and the streaming quality is not acceptable. Buffering times between tracks are problematic.
Shawn then points out a patent granted to Apple to solve this streaming problem. I would like you to consider the smartness of this patent:
Due to a recent patent, it looks like there will be little to no buffering pauses due to combining snippets of songs stored locally with streaming of them. If you synced only the first 15 seconds of your music you could store 20 times more music on your iPod than if you were syncing entire songs.
15 seconds of each track would be stored on your device and would be played while buffering. The result is no silence between two tracks played from the cloud and that's brilliant.
Shawn's article is not only valuable for this little detail. It is worth reading to have an overview of the different services. I therefore recommend it to you.