T-Mobile Borderless Cell Plan - is this too good to be true?

I have been with Wind Mobile on there $35.00 plan in Canada since last November. When we go south I add on there unlimited US Roaming for $15.00. I bought an unlocked phone so I could switch back to one of the big ones in case it didn't work well for me. So far I have been happy the coverage in our area and in USA they are also with AT&T T-Mobile. I do like the fact that I keep the same number when in the US and I pay in Canadian $s.