If you're a T-Mobile customer, you can call Canada and Mexico from the United States and vice versa without extra cost

I remember when my son made some cellphone calls to friends while traveling through Canada on the way back to school in the state of New York.  He quickly learned the practice of phone companies levying big roaming charges on calls made from Canada.  It was a learning experience.

Now, T-Mobile announces that they will not charge anything extra on any calls made within North America, including Canada and Mexico.  

I'm curious.  Are there any catches?  Does T-Mobile have good services in both countries?  This sounds like a good deal.  Agree?  I'd be anxious to hear experience with it.