Research in Motion (RIM) launched BBM Music this week, a music-sharing app a spokesman for the company has said evolves the mixtape. The app went from beta version to public availability throughout Canada, the U.S., and Australia on Tuesday, with plans for more expansion in more countries planned for a later date.

BBM Music focuses upon the idea of sharing music with a specific network of people, seeking to join the ranks of other viral music apps like Spotify and Rdio. Seeking to capitalize upon - and perhaps revive - its BlackBerry Messenger network of 45 million users, RIM's newest app allows users to choose 50 songs from an extensive cloud-based library of content licensed from music giants Universal Music Group, Sony Music Entertainment, Warner Music Group and EMI, as well as from smaller independent labels. BBM users then post their library to their profiles for a subscription of $4.99 per month, giving their friends access to their library and gaining access to all of their friends' libraries in turn. BBM Music normally includes a 30-day free trial, but is currently being offered for an extended 60-day period. The songs are cached on an SD card, so users have access to them even in the absence of wireless coverage, and users may swap out up to 25 songs for new selections each month.

While the viral sharing aspect of the app has certainly found fallow ground in the marketplace - the recent uptick in the number of apps like Spotify, Rdio, and Rhapsody demonstrate its appeal - experts are skeptical that the actual user experience of BBM Music will be easy enough to make the app a success. On the customer's end, the value of BBM Music lies in an ever-expanding network of BlackBerry users whose libraries can be accessed at an increasingly diminishing cost. Critics point out that its reliance upon the BlackBerry OS, whose market share has shrunk considerably from what it once was, is limiting to a degree that may completely forestall the network's ever getting off the ground.

If you're a BBM user, will you be subscribing to BBM Music? Tell us in the comments section.