As a long time Spotify user who switched to Google a few months ago: I really like the playlists and I've found a lot of new music that I like. The recommendations have worked really well. I'm not looking back.
Just checked out Google Music on Wikipedia. No native desktop clients. Not even for Windows, and I use Linux. They can't be serious. Total dealbreaker.
I tend to always have a browser open so a desktop client doesn't matter to me. I keep Play Music open in a tab. Then I use an extension[0] to add the other controls I normally use.
