[kelnos]

It is pretty horrible for latency. Despite T-Mobile's free international roaming (albeit at reduced speeds), I'll usually get a local SIM card if I'm in the country more than a week because the latency can make it unusable sometimes.

IIRC the carriers do this for billing/metering reasons; otherwise they wouldn't have any idea how much data you're using while roaming, and would have to blindly trust whatever the partner carrier tells them.