Recently got a chance to test the service in Mexico, it works everywhere you'd expect a phone to work.
I used my phone a on cell towers handful of times and was billed $0.20 per minute for that. The 4 or 5 calls I made added an additional $2.40 to my bill. Calls back home over wifi were free. Texts are always free when overseas, which is awesome. Data is always $10 per GB in the hundreds of countries that Fi works in, which is an awesome deal.
With low data usage, my bill usually is somewhere between $24 to $30 per month, and averages $27. I usually keep data turned off, and only turn it on when I need it.
Compared to Republic wireless I'm paying slightly more (my bill used to be $24 for Republic with 1GB of data). So for $3 more per month I get to use data and make calls on cell towers when overseas. As someone who travels fairly frequently this is well worth the slight price increase for the convenience.
Finally, if anyone needs a referral code
https://g.co/fi/r/YNRAFK