It's very useful for me. MOST of those whom I text have an iDevice of some sort. I love to see the blue iMessage bubble and the satisfaction of knowing when an iMessage has been "Delivered". Also, the ability to see when the other person is typing, and being able to send whatever messages regardless of the length, pics, videos, etc over WiFi makes it a HUGE win in my book!
Nowadays, seeing the green text bubble, it just feels like it's sort of disconnected from the iOS ecosystem. I can tell that I sent a message but can't tell if it was received, and have to remain conscious of character count. That said, I'm still hanging onto my grandfathered 1500 texts/month plan, and I probably don't use a 3rd of that amount per month.
I have an iPhone 4S. My Mom has a 3rd generation iPad. My Dad, cousin, sister in law, and several of my associates have an iPhone 4, and a few of my associates even still have a 3GS. Plus I got my lady a 4th gen iPod Touch for Christmas last year so we could FaceTime and iMessage.
Poor thing went to verizon early last year (before we met) for her phone upgrade. Curious about the iPhone 4, they suckered her into a motorola droid 2, and the phone has proven to be an absolute nightmare for her ever since. She's since had to get 2 replacement units from verizon due to software issues. It kept rebooting and crashing on its own, and would even send phantom texts to random contacts. Even the verizon guy said he'd never seen anything like it when she took it in the first time! Needless to say, she appreciates the ease, stability, and fluidity of iOS, and basically uses her iPod Touch as her primary media device. Only uses droid 2 for phone calls and to text non-iOS heads. She can't wait until upgrade eligibility comes her way!