These new TMobile One plans are completely un-compelling for me. If I weren't already grandfathered on a previous TMobile plan, I don't think I'd be looking at them now. I have two phones that get 6gb/mo each and maybe burn through 1.5-2gb of that only (since streaming things like Pandora didn't count against the caps anyway). If I'm already locking away 4gb+/mo in my data stash per phone, why would I want to pay an additional $40/mo to be "unlimited"? I think I'd go to Verizon first.
Now, aside from the new $70 starting price point, the international roaming on TMobile is convenient. 95%+ of what I need when traveling will be fine at the throttled speeds (email, google maps, etc). But if you're looking at that $70/mo starting point just for this, maybe you should look at the international prepaid SIMs available at your destination instead.