I would check coverage where you live, work, play, travel, etc. T-Mo does not have good coverage outside of the Boston area, especially into NH and Maine from what I have heard. Northern NH amd Maine is especially spotty.
They do cover the major cities and along major highways so check out all the possibilities.
I have Cingular, live in NH and travel all over New England, NY and NJ and occaisionally points south and west. I carry a Verizon phone as a backup however and have had to use it a couple of times.