I was one of the invitees and am using the service. Although 1/8 of my music has downloaded, I am already listening to my tracks on my phone and on my work PC (my music sits in my home laptop). I am impressed and looking forward to buying music from Google since the experience is so seamless.
Honestly, for what it does, I think it's a great application. It allows me to stream my music to my phone. That in and of itself is a great feature. Eventually, internal memory will become obsolete, and this will be the only/best option. http/www.androidvideoreview.net/2011/05/google-music-beta-android-video-review/
If anyone wants to get their hands on a free Google Music Beta invite, I found a site that was handing out from free. I thought it was too good to be true, until I found an invite sitting in my inbox 15 minutes later.
If you want to try getting your own invite, here's the link to the giveaway: