My first smart phones were blackberrry's, so I have some expereince with BBM. On my BB8800 and BB9000 it was a great option to expand functionality beyond texting on the BB at that time. there is no doubt it is better then straight texting. But eve on BB there was a desire for better IM clients. Many of the same ones are on Android. IM+, Trillian to name a few. Frankly i am tired of the slew of clients to choose from. Hell on the Note 3 we get 4 different software clients with the phone. The built in Messaging app, ATT messaging, Hangouts, and Chat on from Samsung.
Don't get me wrong, BBM is fine, but I don't see it being that big with the saturated space on android phones for messaging apps. BBM's biggest appeal are to those that have had BBM on blackberies and remember how good it was.