it has good aim and im support, blackberries probably work better as an im machine than an iphone does.
you use bbm to reliably communicate with other blackberry users who might not have AIM or yahoo, and might not have texting. it's always on as long as you are connected to blackberry's servers (which is 99.7% of the time) and it's threaded with delivery and read receipts.
for example, my mom mainly uses e-mail, doesn't have AIM or texting, so we just use BBM to communicate short messages that don't need phone calls.