Actually most of the first 7-8 gigs went pretty fast just using phone apps such as you-tube, streaming radio and TV
You have to be an EXTREMELY heavy user to go through 7-8 GB's fast on the phone itself with no tethering.
I know data can go fast but unless you're constantly streaming for hours everyday then it doesn't go that fast from the phone. I own a Slingbox, have a Netflix subscription, a Hulu plus subscription, and a Rdio subscription and I still can't use up that much data in a couple of days. And I use each app everyday. Netflix/Hulu being the exclusion after switching to T-Mobile since there not Android compatible yet. But even on AT&T I would use more than my fair share of data and in a month would barely touch 8 GB's on my iPhone.
All I'm saying is just watch the usage and don't be a repeat offender because you're just asking to get flagged... unless you're using T-Zones in which case the data isn't registered.
But I guess it doesn't really matter since T-Mobile doesn't charge overages, the worst they will do is just throttle you.