SMS is nice, and yes, it's a lowest common denominator that reaches all, but the seamlessness of having iMessage when it's available it a huge plus for me. And I've found that I'm sending WAY less SMS/MMS messages now, and I'd say 90% of my text traffic is now iMessage. But I think this seamlessness (on the iPhone anyway) is a huge draw over BBM: with the former, it goes through iMessage if it can and if not, SMS is the automatic fallback. With BBM, you *have* to choose, you HAVE to know if the recipient has a Blackberry, you HAVE to know their PIN and put it in for it to work.
I didn't think iMessage would be that big a deal for me at all, but with most of my friends/colleagues having iPhones now, I've used it quite a bit. The return receipts are nice, significantly more useful than I thought they would be. Sending over work/home wifi completely cuts AT&T or whatever wireless carrier out of the picture, which I think is also a nice plus. And group messaging is fast. Again these are all things I thought I was fine doing without, but now I'd definitely miss if I couldn't use iMessage.
And, for me, it's BIG deal to be able to send/receive messages on multiple devices. My phone, iPad and Mac can all be used for this purpose, and that's an awesome thing.
For the longest time, I've wanted to be able to compose texts on a computer, or some other device "synced" to my phone. Because no matter how good a mobile keypad is, a full size keyboard will always trump it and make sending messages that much easier. This is something I haven't been able to do since I hooked up my old Motorola E815 to my desktop via serial port, and tried to send/receive message that way... very awkward.
But now, there's no need to tether my phone to something else to send messages that way. Between the iPad and the Messages app for the Mac, communicating by text has gotten a lot more useful to me.
I still use plain old SMS, in cases where I'm texting to someone without an Apple-based device, but the difference is totally noticeable. it's hard to explain, but you see your message in green, and there's a kinda limiting feeling to it. And the loss of that "delivered" confirmation is a bigger deal than I thought it would be.
So, I get now, why Blackberry users loved BBM so much. I still would never use BBM because I find iMessage (and just about everything else on iOS over Blackberry) to be better, but I think I now understand the draw it had.