It's a nice move to be able to call to Mexico from the USA without extra charges. There has been a monopoly by telecom operators to charge a huge amount of money to Hispanic people. On another note when their network work, it is crystal clear, but when it does not work, depending on your location it can be a real pain.
I got some technical issues with T-Mobile in my area (78728) with slow speed of 0.5 mbs, but they sent me a signal booster for free. In other areas (78729 for example) I can get 30 mbs off their network and 8mbs in upload. It's pricey too, but still was the most affordable telecom operator compared to AT&T or other companies.
I use my phone more and more for data than voice, it's the big trend of 2015 and it's going to be this way until voice operation disappears slowly but surely.
I also need to rely on T-Mobile for international roaming when I go to France to visit my family, no other network operator was able to do that.