It worked for us in and around Davos, Switzerland last month.

In Spain, Vodafone ES not compatible
 

When I was in Barcelona last month, my phone automatically selected Vodafone ES. Unfortunately, I kept getting an authentication error when accessing anything (e.g., Google Maps, iMessage). When it automatically selected a different provider later in the day and I got data service, I realized the issue could be with Vodafone.

After calling T-Mobile in the US (which fortunately was free), I was told to manually select a different service. Once I manually switched to Movistar, I got great service during the rest of my stay. Too bad the smartphone wasn't smart enough to automatically choose a more compatible service.

Vodafone subs in Europe and Asia seem to accept voice and SMS services and are now blocking data for TMO - there are ALWAYS other carriers to choose in such countries.

On an iPhone ensure you have a 3G or LTE (select networks only) showing to ensure data is active

On various Android phones it differs; best to do a *#*#4636#*#* in the dialer and select phone info or device info and see that GPRS says "Connected" - will also list type of service under Network Type:

If it doesn't show data then ensure "Data Roaming is on" ensure, on the phone, on the account, dial #RON#
If it still doesn't work pick another network

To help others if you find a network working for data that is not in the Wiki please edit the wiki or highlight it working here. Same on networks that let you connect for voice and SMS only BUT NOT data ; we want to list those under problem networks.

Back to OP: There was a concept of preferred networks on a SIM card eons ago. NO carrier seems to use this now; this allowed the home carrier to do a SIM update populating network number ID's of preferred roaming networks so those got picked over other networks first. No US carriers seem to be doing this and it looks like most smartphone radio software isn't reading this anyway... SHAME!

Has anyone used it in Austria or the Czech Republic? Going to Vienna and Austria next month.

You can direct call Intl support +1-505-998-3793 and ask for who the roaming partners are - this is a FREE call while roaming also.

- Austria: Three, A1, T-Mobile Austria
- Czech Republic: Radiomobile/T-Mobile CZ, Vodafone CZ, O2

I will add these to the Wiki. I would suspect that Vodafone CZ MAY be blocking data like other Vodafone networks.

PLEASE report back on if these worked or if you had issues

Thanks, I'll look for those. Heard that A1 and O2 have the best coverage in those countries.

WIKI Updated
 

I updated the Wiki to clean up some old links and info

Also added the direct dial international roaming support number. This is a FREE number if roaming to T-Mobile support.

For voice I only had coverage in larger cities in Czech Republic using Vodaphone. IIRC this past visit was 02 which was much better.

I've successfully accessed T-mobile sister networks--voice and data--in the following countries not on the Wiki. Just found this thread, so I don't remember the names of most of the networks. Others will have to fill in that info, but the service does work at no additional charge in these countries:

Indonesia
Israel: Orange; CH
Singapore
Korea

I used my new IPhone 6 with Tmo in France, Italy (more Tre than TIM), Switzerland and Belgium. It was a bit erratic, with Facetime working incredibly well but some sites very slow to load (and draining the battery a bit). All in all a very happy camper relative to my days with AT&T and their expensive data that did not seem much faster in the same locales. On occasion my Italian SIM (Vodaphone) was significantly faster on my other phone.

FWIW, Italian Vodafone is easily the fastest SIM I've ever worked with.

If T-Mobile works 1/2 as well in Bourdeaux, I'm a happy camper.

Will report back mid-Nov.

For CH, you meant Cellcom in Israel?

Thanks, so you're confirming both TIM and Tre worked? Please confirm back and I'll put them in the wiki

Roaming worked in both Iceland and the Faroe Islands this past weekend (10/17/2014-10/20/2014). I had purchased a data pass and got good speeds in both locations -- definitely higher than 2G and very usable for everything I use my phone for. I was even able to successfully tether my tablet through the connection.

In Iceland, I roamed onto Nova. There were occasional periods of outage or spottiness but overall it worked well enough.

In the Faroe Islands, I first roamed onto Vodafone, which worked for calls and SMS but not data. A few minutes later I noticed that the phone had re-roamed onto Foroya Tele, and everything started working. In fact I never experienced so much as the tiniest glitch in the Faroe Islands, it was very reliable everywhere I went.

Windows Phone 8.1 Update 1 on both Lumia 925 and 1020, automatic network selection.

Thoughts on having a section in the wiki regarding places where a data pack is strongly urged?