I travel between the US and Canada and I find T-Mobile's roaming pricing liberating. They already used to have unlimited data and texting when roaming internationally. Now they've made calls unlimited as well when roaming in North America. Contrast this with say ATT which charges you through the roof for roaming, I'm wondering what about T-Mobile allows them to offer free roaming.

Unlimited international data is 2G. The new North America plan offers 4G within your plan's limit. Note that you need to be upgraded to the new plan, it's not automatic.

In the last few countries I've been to, I've gotten 3g while roaming, which is faster than T-mobile promises.

Yes, I get 3G as well. In Canada, I already used to get LTE when connected to Rogers