Google Music has gone live, roughly six months after it launched in beta and ages after related rumors first began swirling. The U.S.-only version unveiled today takes that earlier cloud-based personal music library service and adds multiple device synching, purchasing, Google+ integration and exclusive content. Much more interesting, however, are the ability to share songs with friends and the new Artist Hub feature for musicians who want to control their own sales and  relationships.

Each Google Music user can store – and access from anywhere – 20,000 songs for free, regardless of how the tracks were acquired.  That competes directly with Apple’s iCloud suite, which costs $25 a year and can handle unlimited iTunes-purchased songs but maxes out at 25,000 songs from elsewhere. Being able to access a cloud-based personal library from anywhere is now available to the 44.8 percent of all U.S. smartphone users on Android , instead of being limited to the 27.4 percent on iPhones (as reported by comScore).

Jamie Rosenberg, Google’s director of Digital Content, particularly enjoyed pointing out the price differential during the launch event in Los Angeles, saying, “Other cloud services think you have to pay for music that you already own. We don’t.” He was speaking at a launch event in Los Angeles that Google had labeled, “”These Go to Eleven” in honor of Spinal Tap’s legendary amplifiers. Apple has the rejoinder that Google Music users have to upload their entire library instead of the service matching songs to tracks already in the catalog, a difference between which consumers can choose for themselves.

Sharing songs comes via Google+ integration. Users can send any song they purchased from Google to anyone else on Google+, and the recipient can play it all the way through one time. Microsoft pioneered that idea with its ill-fated Zune, but it has become more useful now that the capability is web-based and in a more widely networked marketplace. This technological progress also makes possible the Google Music capability for users to stream an entire album to another user.

Sharing also adds to music discovery, as does the Google Music Store’s inclusion of charts, rankings, promotions, staff picks, featured artists and other curated guides. The Google Music store offers more than 13 million tracks from artists on Universal Music Group, Sony Music Entertainment, EMI, and over 1,000 independent labels including Merge Records, Warp Records, Matador Records, XL Recordings and Naxos. It also has a deal with independent rights agency Merlin and digital distributors of independent music including IODA, INgrooves, The Orchard and Believe Digital. Conspicuous by its absence from the list of partners is Warner Music Group.

To help promote Google Music, users are given a nice selection of content unavailable elsewhere, some of which is available for free download. For example, The Rolling Stones are offering an exclusive, never-before-released live concert album, Brussels Affair (Live, 1973), and a free single, “Dancing with Mr. D.” Other exclusive free live recordings include: Coldplay, “Every Teardrop Is A Waterfall”; Shakira’s live EP from her recent concert in Paris; Pearl Jam’s album from their 9/11/11 concert in Toronto; and a selection of Dave Matthews Band concert recordings.

Free exclusive studio recordings include: “Why Stop Now” (feat. Chris Brown), the first single from the upcoming Busta Rhymes album; “Je L’Aime à Mourir” by Shakira; and  “What Can We Do?” (feat. Anastacia), a new mix from Tiësto.

Musicians who want to maximize their fan connection may be interested in the Artist Hub, which for $25 lets them have their own store by setting up an artist page with bios, photos and whatever else they want, then upload and sell tracks, choose if they want to allow previews, and set their own prices. Google takes a 30 percent share of any sales but there are no other fees.

Related links:

Google blog post by Andy Rubin, senior vice president, Mobile –