I've had an iPhone since the 3G with unlimited data and I've only gone over 2 GBs once and that was during last year's NCAA March Madness. The devices only destroy your data if you sit around and watch Netflix and tether all day. Standard browsing and watching a few videos will keep you under 2 GBs.
I dont tether or watch netflix. I listen to Pandora, Spotify, and Stitcher in the car. I get email from 4 addresses, I chat, I surf, I use facebook. I use my iPhone all the time, and maybe use 60 mins a month. I can't remember the last time I watched a video and tethering is another fee and has nothing to do with the unlimited data anyway.