I recently got around to checking out Google Music and I was pretty impressed. The idea of being able to have my music on the cloud and the support of Google became more attractive now the the Nexus wont have a SD card.
With that said, I've found songs to be more expensive in Google Music than Amazon. I only noticed because when I hear a song I like on Pandora I add it to my wish list in Amazon. (It would be nice if Pandora added an option to purchase on Google Music.)