Just got back from mexico (cancun, isla mujeres, etc.). I had useable 3g mostly everywhere i went via Telcel and Movistar partners.

Just turn on your "roaming" and you should connect to a T-mob partner that is included in the worldwide coverage. If it doesn't connect to something seemlessly, chances are there are no reachable providers or it's the provider is not included in the "free worldwide coverage".

This was a good "simple choice" move by T-mob.

I just want to add that if you call "Tmobile Support", chances are you will hear a million variations of how to use it.

Just turn roaming on and enjoy.