I just wanted to share my experience so far -- both Google Hangouts with my US GV number and the pre-paid UK sim from EE work flawlessly in Europe. Furthermore, the mobile data I get from EE is much faster than the crappy free WiFi Internet that I get from not-so-cheap hotels in Germany and France. So, I would highly recommend this combination to any occasional US traveler to Europe.
A few points that some people could find helpful:
- Even though EE customer service reps claim that their sim cards could only be activated on the actual EE network in the UK, it's not correct. My EE sim card was activated in the USA a few minutes after I popped it into my phone. Of course, I couldn't get data or voice there, but I could receive text messages just fine.
- Google Voice number with Hangouts work fine no matter where you are, but when you are using a foreign SIM card and want to call a 10-digit US number from Hangouts dialer, you have to prepend it with +1 (e.g., +1 234-567-8910).