The "free roaming" is EDGE (aka "2G") [1], which is too slow for most things these days, to the point of being unusable. You could check your email in a pinch, but that's about it. Forget about, say, Google Maps.
I'm in Germany at the moment, and I'm using a European SIM card instead of my T-Mobile in order to get stuff done. Germany's 4G/LTE is notoriously spotty, but at least I can get 4G most of the time.
You can buy T-Mobile "data pass", but it's ridiculously expensive; the largest pass you can buy is a 500MB, for $50 (!).
The competition is equally bad, of course. AT&T charges $10 per day (!) for international roaming, meaning you're given the same amount of data as in the US.
I've been using the free unlimited roaming from T-Mobile on several trips to Japan, Argentina, France, and Italy. Often, the speeds are actually faster than EDGE, unless the network is busy. I have had no issues using Google/Apple Maps or booking hotels, etc. through this connection. The only thing I miss is having Hotspot capabilities, even at the reduced speeds.
EDGE is consistently, unbearably slow for me. If the phone has negotatiated EDGE, you should be getting EDGE speeds, which has a maximum bandwidth of 236 Kbit/s (theoretically up to 473.6 Kbit/s according to Wikipedia). That's not fast. I occasionally get reduced to EDGE on the Berlin subway, and I barely get Hacker News up.
(LTE has a maxmimum of about 450 Mbit/s, but in practice it's closer to 15-50 Mbit/s down, maybe half that up.)
Maybe someone who knows mobile technology could chime in.
I am a big fan of Google Fi. It's $10/GB pretty much everywhere in the world. Really great if you are hopping around to different countries. Also you can get additional data only sims for no charge.
I used T-Mobile for a trip in the past and found it to be pretty frustrating.
it's actually LTE, just rate limited. With t-mobile one plus, you'll peg it at its max of 256kbps (or 32KB/s). While that's painfully slow for web browsing, it works perfectly fine for online taxis, directions with maps and even e-mail and chatting. i.e. anything that uses the internet passively.
"Perfectly fine" has not been my experience anywhere. A months ago I arrived in Oslo from the airport, and tried both Uber and the local transit ticket app, and both were impossible — everything just timed out — so I ended up hailing a cab the old way. You'd think the connection would be slow but that it would eventually get you there, but that has not been my experience.
T-Mobile's free roaming worked great for me when visiting Russia earlier this year. WhatsApp text messages, email, Google and Yandex maps, taxis, schedules etc. were all completely usable. Particularly funny since I was getting good connectivity far in the countryside, in the woods, in the sort of area where in the USA one might expect no coverage from any provider.
Uploading photos was a pain point though, and a major one: 2-5 minutes per image.
i've used it in turkey and israel without a problem within the past few months, web browsing was mostly a no go, but passive things worked.
One thing that I would add is that you have to really internalize what it means to have a severely rate limited connection. You have to control your sync settings. if you just landed and all your services are trying to sync, you'll be using all the bandwidth and everything wont work.
I'm in Germany at the moment, and I'm using a European SIM card instead of my T-Mobile in order to get stuff done. Germany's 4G/LTE is notoriously spotty, but at least I can get 4G most of the time.
You can buy T-Mobile "data pass", but it's ridiculously expensive; the largest pass you can buy is a 500MB, for $50 (!).
The competition is equally bad, of course. AT&T charges $10 per day (!) for international roaming, meaning you're given the same amount of data as in the US.
[1] https://www.t-mobile.com/travel-abroad-with-simple-global