Ok, so....

Thinking about switching from ATT to T-Mobile solely because of their international data plans ("unlimited data in 120+ countries").

Let me start by saying: I love ATT in the US. That being said, their international data plans are a joke. I ended up unlocking my phone because it's easier/cheaper to use SIMs than to pay for text ($30), data ($60), and call ($30).

Regions I travel to often: Sicily, Netherlands, SE Asia, S. America.

Travel coming up: Egypt, Thailand, potentially Iran.

Anyone have experience switching from ATT to T Mobile? Any specific issues?

My main concern is I'd be losing my grandfathered status for unlimited domestic data....

EDITS:

1) Phone: iPhone 5S

2) Not obsessed with "fast" data...just need basics, i.e. email, iMessage, WhatsApp, Instagram, Facebook, and occasional web browsing. Every so often I need to use Google Maps....