How are they doing that?

If you’re out of the country for that long of a stretch at once, I wouldn’t recommend T-Mobile (I mention it above). SIM cards will work out cheaper. T-Mobile is good if you are bouncing in and out of the US.

For those travelers an extended amount of time, SIM cards will definitely work out cheaper.

T-Mobile is 3G unless you pay for higher speed. AT&T isn’t free!

That’s awesome to hear!

How does the quality differ? It appears from comparisons on websites that AT&T can do LTE in Europe, but T-Mobile is 3G at best. But the web sites aren’t consistent.

So glad we switched. Don’t have to worry anymore about turning off phone or will rack up huge fees.

However, there are some zones or conditions when Google Fi would be better:
1. For a globetrotter, Google Fi’s rates drop dramatically when placing calls from a non-Nafta location if you connect via WiFi; in contrast, T-Mobile charges 20 cents per minutes to place calls from outside NAFTA EVEN IF YOU ARE CALLING OVER WIFI.
2. If you travel to countries not in the “country bundles”, Google Fi is cheaper when you are using over WiFi; T-Mobile charges the same rate whether or not your are using WiFi Calling.
3. For data speeds, Google Fi offers at least 3G speeds internationally (and, in future, 4G); T-Mobile only offers 2G! You may even be able to place Google Fi calls over 3G/4G and not pay the 20 cents per minute for cell network calls!
4. For data cost, the theoretical unlimited T-Mobile plan is of little use outside of NAFTA since you can at most use it for e-mail. Google Fi charges a flat $10/GB AND unused data allowance rolls over!

The long and short – if you are mainly in NAFTA countries, T-Mobile is unbeatable.
However, if you are travelling through other countries on a regular basis, Google Fi would give you faster data speeds and potentially cheaper voice calls.