I am still under my grandfathered unlimited data plan from when I got the original iPhone back in Feb 2008.
Now I sit on my nearly 2 year old iPhone 4S, patiently awaiting the release of the 5S. I have no plans to part with my unlimited data plan. Even though keeping unlimited apparently means I am not allowed to tether, nor am I allowed to FaceTime over cellular, I don't care. It's not worth giving up unlimited.
That said, with the 4S, I really don't go over 2-2.5 GB per month. However, when I get my hands on the 5S and it has LTE, I can see how things may change.
My Dad still has his unlimited data plan too, from the iPhone 3G back in 2008. He is currently on the iPhone 5 (he's NOT on the "S" cycle) and has retained his grandfathered plan. Neither of us plan to part with unlimited.
At least AT&T (for now anyway) allows grandfathered customers to upgrade for subsidized pricing and still keep their unlimited data plans. My lady, who is on Verizon, had to give up unlimited and go tiered (2 GB/month for $30) when she upgraded from her Droid 2 to a iPhone 4S last Oct. But she barely uses much data so it made no real difference to her anyway.