Two things:
1. Someone asked about roaming partners in Mexico. As far as I've seen around in the Internet I think it is both Telcel and Movistar. My son is traveling in Mexico and he saw his phone change from Movistar to Telcel LTE (but not higher speed). Perhaps this is in preparation of July 15.
2. I've seen a lot of media coverage, forum posters, AT&T apologists fail to realize that even bigger than the roaming functionality it is the free calling to Mexico and Canada. With millions and millions of people making calls to friends and families in Canada and Mexico, T-Mobile has a good chance of getting a lot of people switch to T-Mobile. I've seen forum posters saying that AT&T might not be forced to act because this plan attracts only business people who travel to Mexico or Canada frequently. As I say, they fail to see all the regular folks who call to Mexico often and also travel with family to Mexico or Canada frequently. I understand Mexico is also (through legislation) moving towards free calls from Mexico to the US. One big player was Nextel (with their 2-Way radio function) that allowed people to communicate from Mexico to US and back for free. Now there are many more options to communicate for free (voice, messages), WhatsApp, Skype, Messenger, etc. However, for an old good call to a fixed line, this T-Mobile offer is hard to beat.
I was an AT&T customer for years, but I am having much more fun with T-Mobile and their antics than I ever had with AT&T. Yes, AT&T has better coverage, but they lack innovation (Wi-Fi calling, they were late on Voice over LTE, etc.) and I really dislike their overage strategy. I much rather suffer an occasional (not often at all, as I live in a large urban area) poor signal than AT&T business practices.