A data point from first hand experience:

I already got to use the plan in Japan, Hong Kong and Taiwan.
It worked without a hitch in all of the above.
The connection speed (shows as 4G, but is clearly throttled) is enough for all email access, chat (google talk) and the text messages work without problem. Even Google maps (without satellite view) is doable without difficulty.
Usage of data shows on the account as overage but with $0 cost.

Very happy about this new plan. It means I do not need to hunt for a local SIM anymore in most places and I will on occasion buy one of the fast speed packs!