The difference between iMessage and BBM is reliability. Anyone who has used iMessage on multiple devices will no doubt attest to the delivery issues.

I'm fairly ok with Keynote presentations taking time to sync down or issues with photostream syncing but when a message from my wife asking me to pick up the kids from school comes in three hours later then I have a massive issue with the product.

Why does apple find it so hard to get online services right?

Ironically in my experience BBM 'just works' the way Apple products used to.

By the way I would not be surprised to see Apple blocking BBM as it duplicates existing functionality on IOS