I suggest that anyone choosing to rent a car in Germany should browse through the experiences of those who have gone before. It's not just like renting back home, so rent with your eyes wide open.
Let me chime in with my experiences... done it, no problems at all. Actually, I've had less problems renting in Europe than in the US. The only major caution I would give, so that you avoid unwanted "souvenir photos" of yourself driving the rental car (from speed cameras) is to understand the rules of the road regarding speed limits. Know not just the speed limits signs, but the rules for when the default rural and urban speed limits apply. And realize that unlike in North America where the police have the discretion to only pull over the fastest, most reckless drivers, speed cameras will flag everyone going over the limit.
I should also add that I have found Sixt to offer the most superior service of any rental agency, anywhere. They even sent a representative to a small Swedish airport to meet me late at night, long after their office had closed. The problems listed in the linked reports above seem to be related to not understanding the conditions of rental.