I installed it on my Nexus 4. Google Music just needed an upgrade to get this started. I signed up. I then did the same thing on my Nexus 7 and it already knew I was signed up and was ready to go. I'll likely use it on the Nexus 7 the most because that's what I listen to music the most on.
Glad to know I'm not alone!
It's a great app; I love that I can stream my personal collection.
of course this app isn't for everyone but no one is making them use it