My household has 7 in all, and there is rarely a time when any of us are home that we aren't playing Minecraft, WoW, LoL, SW:TOR, streaming Netflix or YouTube in HD, listening to Pandora, etc. My daughter, with a multi-monitor setup, will play on LoL while watching Netflix and videochatting on Skype at the same time, for hours on end. I've topped the 250GB cap every month since December, and have gone as high as 300GB. I am the type of user that the current cap* and this new tiered plan is targetted at, and frankly, I think it is fair.
My mother-in-law also has Comcast, and she turns on her computer and checks her email maybe a couple times a week. She helps give Comcast the statistic that the "average" user consumes less than 5GB per month. She also pays almost the same as I do per month for her internet.
*: Oddly, I haven't heard a peep out of Comcast about exceeding their cap, probably because I'm paying them over $200 a month for a bundled plan with things like a landline that I don't ever use, but had "just in case," and cable service that isn't even hooked up to any TVs, but that I keep so we can watch HBO shows on-demand online. I'm already planning to ditch the phone and TV service completely, and just pick up seasons of a handful of shows on blu-ray. Once I can find that last set top box I have to return...