CNet columnist Marguerite Reardon has declared, “The new iMessage feature in iOS 5 from Apple will certainly be a BlackBerry killer.” I would certainly temper that expectation.
iMessage, from what I’ve been able to see from the YouTube videos, does the same thing as other apps already in the marketplace: WhatsApp, LiveProfile, and PingChat. Those apps also have cross-platform clients, something iMessage won’t be. The iPhone users who were already looking for a way to get around SMS charges probably already have one of these apps.
BlackBerry’s own messaging system, BlackBerry Messenger, also benefits from encryption, whether going through RIM’s BlackBerry Internet Service servers or a company’s BlackBerry Enterprise Server environment.
That’s not to say that Apple can’t make a better iMessage app than what’s already out there. Our experience with the cross-platform applications is mixed. All three systems have dropped messages, and only PingChat has told me that a message hasn’t made it and let me retry.
If RIM’s BBM were cross-platform, the game would be over. 
Not sure it’s a killer of anything, but my understanding is that the big difference is that it’s integrated into the messaging app. It’s Apple at its best. You don’t have to choose whether you’re going to send an SMS or use iMessage it just knows whether the other side is capable of iMessage and uses it (not sure if there is a way to override and choose SMS). I think most people aren’t looking for a way around SMS, but if they have a large enough network of iOS users they’ll see their SMS numbers drop and then think about changing their plan.