I never really understood the BBM, because I never had enough friends with blackberries to make it worthwhile.
But in most cases, its just another form of chatting like AIM. They say its "completely" private, but I don't see how its any more private than texting/instant messenging.
This app sounds nice though! Definitely would like a way to chat with other people without giving them a phone number! Not many people use AIM these days.