If you do a lot of traveling in the EU from the US, T-Mo is about the best option short of buying a sim while in the EU. Unlimited texts, data[1] and very cheap calls are awesome. I never think about my phone with traveling now and just use it like I'm in the US. The only downside is there are some countries in eastern EU not yet on the plan.

[1] The data isn't 4g, but for travel related items like TripAdvisor, maps, etc... it has always worked fine for me.

If you buy a pay as you go sim in the UK and use that when you're in Germany, you'll benefit from the existing roaming charge caps.

This is about roaming _within_ the EU.

http://en.wikipedia.org/wiki/European_Union_roaming_regulati...

make sure your phone is unlocked and buy a pay and go sim card here. You can easily do your research before you leave to find out any necessary details.