Since switching to T-Mobile's Simple Choice we've used it quite successfully and economically in Canada, France, Germany, Italy, Netherlands, Switzerland, Austria, Hong Kong, Sri Lanka, Malaysia, and Vietnam. The coverage is often better than it is in the U.S.
We've added the high-speed packages twice but don't feel we really need them for our usage pattern.
We've had a few hiccups along the way, especially going between Wifi and cell towers, but rebooting seems to help.
We also used it recently for a couple of Wifi calls at a resort in the Maldives
after I had turned off data, and I was shocked to find a bill for over $300. It's my fault for not reading the website carefully enough (and the TM phone rep had to research it herself), but they reversed the charges and are charging me 20 cents a minute for the calls. ^^
The rep confirmed I could get around this by using Skype on Wifi in non-Simple Choice countries. I do that whenever convenient to save most of that 20 cents per minute.
For us as leisure travelers T-Mobile's Simple Choice has been tremendous and we've found the phone support to be excellent.