Not only will you know if some one you are corresponding with is writing you a response, you can even tell if a message you wrote has been delivered and read. BBM is open 24/7/365 so there is no need to open the app. And the famous BlackBerry PIN system means that your phone number or email address do not have to be given away.
Right now, BBM allows you to chat with up to 30 users at a time and you can share documents, voice notes, files, photos and more. Eventually, BBM Video Chat and Screen Sharing will be available with an update down the road.
source: BBM (iOS|Android)