Emphasis mine.
My wife and I got the $30 100 minute/"unlimited" data plan as a stop-gap before leaving the country to move abroad. Before leaving we drove from SC, to MA, to CT, to NY, on over to WI. Unless we were near a major city, our devices were roaming on to AT&T, where we only had 10MB of data to use. The voice coverage was fine while roaming, but since we were using one of the phones as a GPS to navigate, it got annoying at times.
Now this is a cut-rate plan that's full of limitations, and we knew that, but still the service we received was much worse than we expected. I'd strongly advise anyone considering T-mobile to check the roaming agreement.