I regularly hit the 7.5 GB cap usually a week or so before the next billing cycle starts. (Tethering is my sole internet access as I refuse to use Comcast, so...). Speeds hover somewhere between 10 and 20 KB/s, though it fluctuates which is more problamatic than the speed itself. I can still surf while tethering, though stuff like Youtube is obviously out of the question, though I wonder if I could download a mid res video overnight and watch it later. Usable, but have something to keep you occupied while waiting on heavy pages. Oh, and Google Maps still works fine at the throttled speeds.
It's still quite superior to dial-up btw, which I had the pleasure of using recently at a relative's place. Goddamn, I may as well have the mail truck deliver forum prints instead. 2g sucks as well, even though it's rated at throttled speeds, consistency is far, far worse (50% of the time, I time out.)
Funny enough, Verizon gets absolute crap service where I live, but where I work, I get nothing with T-Mobile. Since tethering at home was important, T-Mobile was a quick choice.