Also - NYTimes just posted an article about this issue:
http://www.nytimes.com/2016/05/26/te...ne-abroad.html
Frankly, not really informative, and they, like many here, give T-Mo the brush-off. Seems pretty clear that the author never even tried it.
Also, don't forget that if you're looking to pay for high-speed international roaming, T-Mobile is cheaper than AT&T ($60/300MB) and Verizon (complicated, but expensive). It's $50/500MB. Only Google Fi is cheaper ($10/1GB).
I haven't tried Sprint's free roaming, but T-Mobile has been fine for me.
Has anyone tried Sprint?