BBM has been the killer app because it is a great alternative to SMS, it offers group messaging, integration with a number of other BlackBerry apps, and now it offers voice calling between users. The update is available in the BlackBerry App World, and can be installed on any BlackBerry device running version 6 or higher, but should come to handsets running version 5 next year.
BlackBerry may not have much of a market presence left in America, but internationally, especially in developing nations, it still holds a solid share because of the integrated data plan. International messaging is an especially costly endeavor, so this update should prove quite useful.
source: BlackBerry App World