As I understand it, basically. Granted, I've never used BBM before.

If you try to text someone with iMessage, it will send them an iMessage instead of a text (assuming you have iMessaging capabilities). Aside from being free, the only other real difference seems to be typing notifications.

There is also upcoming/beta desktop client integration coming with Mountain Lion, fwiw.

Except I don't think BBM ever made text messages look like BBM messages. Does iMessage show you a "read" mark like bbm or what's app?

I would agree that any device only messaging system (BBM/iMessage) are probably things to stay away from if you value an easy future choice without telling everyone to stop using the old method.