One area I hadn't considered in my previous post, iMessage vs BBM, was how this will affect international communication.
With most cell phone providers you pay a lot more for international texting rates than you do for in-country texting. Now, if your friends or family have an iOS5 compatible device those costs are gone. There's no fees associated with sending an iMessage to another iMessage compatible device.
In yesterday's recording of Lost & Lemon (not out as of yet), my brother in law in Australia was asking if he should upgrade his iPod touch to iOS5. Aside from stuff that's neat or fun, I couldn't come up with a really compelling reason to upgrade right away - eventually, sure, but no rush. iMessage could change that in the same way that FaceTime did for iOS4.
Macworld has a great writeup on how to set up iMessage but one thing I like is how iOS5 makes it easy to tell if you're sending a text message or an iMessage - the message entry field tells you as well as a colour change:
One other feature that's new in iMessage is the ability to swipe down on your messages to get rid of the keyboard. Handy if you're in the middle of a longer conversation and can't remember what's been said.