I have an upcoming trip to Europe visiting both Germany and Spain, and am renting cars from Hertz in both countries. I found it interesting that the conf. email from Hertz for the Spanish rental specifically mentions that an IDP is required in the fine print, whereas the conf. email for the German rental says nothing about an IDP.

I have never gotten an IDP for driving in Germany. However this time, since I'm going to Spain too, I got one. Takes 15 min. at NorCal AAA; cost is $15 (AAA member). They'll take the required passport photos too if you don't already have them for an additional $10.