Interesting. I've used Beluga, WhatsApp, Brightkite and GroupMe, none of which were ever truly satisfying, mostly because there's no strong network effect in place - anyone I messaged with these apps was someone I bugged to install and use it.

BBM brings to the table a sizable entrenched user base. I agree with the person that said this would have been an iMessage killer a year ago. But this will be crossplatform and iMessage is not. Good for us who aren't living entirely in the Apple ecosystem.