I've been testing the BlackBerry Messenger (BBM) for Windows Phone 8.1 for several weeks. Despite how the industry might feel about RIM as a hardware provider these days, the software piece is stellar. Of course, like any mobile communication service and app, those you want to communicate with have to also be a member of the same service, but the experience BBM presents is probably the best I've tested so far. BBM provides what you would expect: chat, picture sharing, voice notes, group and team creation and management, and more.
Yes, there were bugs during the private beta, but most of those have been worked out to the point that the app is now available for everyone to test. Today, BBM has reached public beta.
You can download it from the Windows Phone store: BBM for Windows Phone BETA
Personally, I'd love to see more of my colleagues and friends join, so I have more people to actually talk to. <grin and wink>