This is an odd topic to pick up as an American in Germany. I've ridden in taxis from around a dozen countries. Some were fairly unsafe (Turkey and Panama, for example.....with holes in the floor of the car noted). When you come around to German taxis.....you become a bit shocked.
First, you will never find a unclean or untidy German taxi. It simply doesn't exist
Second, you just want find any German taxi driver who uses a GPS. You mention any oddball street? The guy might call the desk to ask for a reference point, but he generally doesn't have a problem in finding the address.
Third, fair and honest are a trademark of German taxi operators. I don't know of a single case ever.....where someone had a dishonest situation with a German driver.
Fourth, as you come to figure out after a while.....almost half of taxis you come across in Germany....are Mercedes. My suspicion.....humble thinking of course.....is that they are less of a maintenance problem than the other brands.
Fifth, day or night.....you can show up at any airport in Germany, and there's a taxi sitting there. The same is true for all major train stations.
Sixth, English-speaking? Well....in my experience....around twenty-five percent of the drivers speak some English and you can get your address across easily.
Seventh, and final.....if you were stupid enough to have a few drinks and not in a condition to drive.....that cellphone and local taxi number is a life-saver. Course, don't be stupid enough to throw up in the backseat of the taxi......the drivers get a bit testy over an event like that.
I should add.....after ten kilometers.....this taxi usage gets to be a bit expensive. And if you were going to use them for a fifty kilometer ride.....you'd best make sure you've got plenty of cash on you.