One of the reasons I switched to T-Mobile is because of the unlimited international data plan. Although the speed is limited to 2G, it’s really taken away the stress about how to get data while travelling. I mostly use it to look up directions, message people, and call taxis, and it has been super useful in situations where there isn’t any free Wi-Fi around.
Previously, my international roaming had been working flawlessly on my Nexus 6P, except for the fact that I had to manually select the T-Mobile compatible network when I arrived into a country. I play around a lot with custom ROMs for my phone though, and I had installed a new update a week or so before I left for Panama.