I rented a 5-series and drove from Frankfurt to Schwangau via Munich last December. I've been driving in the US for 4 years now, and can tell that the Autobahn is only as stressful as you want it to be. There are multiple lanes, and if you're in the leftmost lane, you'll be tailgated by those with better cars even while doing 230 kmph. If you're not into that kind of stuff, just stay in one of the right lanes and enjoy the view (otherwise you better stay extremely focused on the road!).
Driving on the Autobahn also made me realize that buying a car for its speed is extremely pointless in the Bay Area (or any place that imposes speed limits), because one would never be able to experience the thrill that comes out of pushing the gas pedal for 30 seconds straight.