Usage based billing sucks. I never really had to worry too much about it before, but ever since we got Netflix and have been purchasing music, games and movies legitimately via the interwebs, I have a hard time sticking to my 175GB, especially since we're three fairly heavy users (well, 2+1 light user) at the homestead.
Paying $1.25/GB is not my idea of fair, especially when it costs pennies to get that data to my house. It's bad enough that my cell phone bill is ridiculous, but generally speaking, if you're going to put a data pipe to my house, I want to be able to suck down as much data as I possibly can.