A few years ago this would have been a killer app. Now I don't know a single person that owns a Blackberry (outside of a few that have them as work phones - and so, I won't BBM them)
Further to your point about cheap, traditionally BBMs wouldn't incur text message fees. These days though it seems like carriers offer plans with a sufficient number of text messages for most people... which isn't a surprise given the huge markup that they've always had.