I see lots of posts asking about various carriers and options for phone and data and I thought I'd report on my recent experience using my T Mobile plan in Europe last month.
I've had T Mobile for years as my domestic carrier, but this year I upgraded to their new simple choice plan so that I could use my Iphone for data while in Europe. This plan includes 20 cent a minute international voice, free texts, and free data roaming (although they only guarantee 2g speed) in some 100+ countries.
i used either phone or data in Frankfurt, Helsinki, St Petersburg, Stockholm, Scotland, London, and Reykjavik. All 6 of the countries that I visited or passed through are included in their plan. In general, data worked well for what I needed (email, looking things up on the internet, using googlemaps for navigation in cities, etc. Sometimes, it would take about 30 seconds or so for googlemaps to come up, but once it was up the response time was good. The only place that I had a problem was on the Isle of Skye where I could never get a data connection and the phone didn't always find a network. In the cities, it worked well. I used the phone to call home as well as make restaurant reservations with no issues. I also used the free texting to meet up with friends.
Each time I arrived in a new country, I got a "Welcome to country" message stating the 20 cent per minute voice rate and unlimited text and web coverage. They would also include a link where I could puchase high speed data at an extra fee. I never purchased high speed data as the slow data was more than acceptable for my needs and I limited my data intensive activities like sending photos to times when I had a wifi connection.
I just received my bill for last month and there were no surprises. I had just $4.20 in additional voice charges for 21 minutes of international calls. The data and texts really were fre as promised.
All in all, I was very happy with the service for the price.