First, let me start off by describing what Google Music is not. It's not Pandora or Slacker. The music that you hear on Google Music is your own music. If you've ripped all your CDs and downloaded a bunch of music purchases, that's what you'll put into Google Music. You do have the ability to purchase music; but, you don't have to repurchase. You are storing your existing music to the web.
Why Google Music?
Two words: syncing hassle.
As in, Google Music eliminates the hassle of syncing your music on your devices. Once you upload all your music to Google Music, you can stream your music to your Android phone (for now) or work computer. This means that you can use any computer to log in and listen to the music you already own.
Your music syncs.
Your favorites sync.
Your playlists sync.
How it works
Once you receive your notification, you log in and choose your preferences. If you have an Android device, reboot so that it knows you are cool now. Then, you can download the app onto your computer. After you install the small app, it asks you where you keep your music. Tell it where to mooch, and it'll go to work uploading your entire library to Google servers.
Most of us have asynchronous Internet connections; i.e., it takes longer to upload than to download. Therefore, it may take a while for your music library to upload to Google. However, as titles get added to the Google Cloud, the titles get synced to your Android device. Obviously, downloading all the music would make your mobile plan cry for mommy; instead, the music gets streamed to your device as you need it.
If you have a phone and a tablet, or two phones, with Android, they'll get all your music too. You can log into your Music account from work and listen to your tunes from the browser without having to install software and annoy your IT department. Your experience is roughly identical no matter where you access your music.
Google has mastered syncing your email, contacts, calendar, and many other important parts of your life. Syncing your music was the next logical step. If your computer crashes, no need to worry about losing all your music or having to copy back from your burned discs.
If you already own a big music library, it may even eliminate the need to subscribe to a music service. You simply buy more music and add it to your Google Music library for access anywhere.