You might want to look at the coverage maps to see whether the operator has good coverage in the areas you plan to visit. Here are the currently registered GSM operators in Germany and their coverage maps.
GSM Coverage Maps | Germany
I´ve read now that T-Mobile or Vodafone are a good choice. I plan to visit Berlin, so I think Fonic (O2) is ok for me. I will stay 2 weeks there, so I think it will cost around 20 Euros for these 2 weeks. That´s really ok.
I have an ipad 2 with wi fi and 3G. I also have an AT&T iPhone 3GS. I have an AT&T voice and data plan for the phone and a 3G rate plan for the iPad (also AT&T). I am in US. Will be traveling in Germany for two weeks next month and want to be able to use these devices for GPS, translation and other apps. For communicating with family in the states and Germany, I plan to use WhatsApp and Skype as much as possible. Except for emergencies, I don't expect to call or receive calls in Germany from the US, although I'd like to be able to make calls within Germany.
Please, someone, give me some advice! I am a newbie all around, from the gadgets to the travel.