Now that Google Fi is available for iPhone and a wider range of Android devices, the fact that it allows use of your high speed data internationally at no additional charge may cause me to switch. I use between 1-2GB/month on T-Mobile. Google Fi would cost me only $30-40/month and I could use my data anywhere I travel.
So far as I can tell, T-Mobile has no plans that compete. Any option for using your T-Mobile data internationally are at 2G speeds.
I hope T-Mobile offers a comparable option before I take my next trip out of the country. I don't particularly want to switch to Fi, but it would be ideal for me.