Apparently, if you go over, you still get 2G speeds.
Quote:
Full speeds available up to monthly allotment; then slowed to up to 2G speeds for rest of billing cycle.

That sounds suspiciously like "free, unlimited 2G service forever, with occasional fast LTE as a bonus". Amazing if true. The fine print suggests you need some sort of active account, even a prepaid one, unused and on a different SIM, will do the job. If you're doing business with T-Mobile anyway, this basically says "adding a tablet is free, if you don't need lots of data". Still great.