Those on this thread might be interested to know that Google just saw T-Mo's bet and raised...
They are now offering Full-Speed "free" data roaming in their 135 countries worldwide on Project Fi... with no end-date defined. I just got back from India where that would have been super useful. This weekend I'm headed to London with my T-Mo sim. I love this new T-Mo promotion and hope they expand it to all covered countries indefinitely.
If they don't, a switch to Project Fi is starting to look very attractive.