Good experience with a minor hiccup at the start. I originally reserved a BMW 2er or equiv, but they didn't have it available. The guy at the counter seemed very excited that he had secured me an upgrade as a replacement - a BMW Convertible. That would have been great if it was just me, but not so much for two of us plus 2 weeks worth of luggage. I had to wait for him to have a friend grab a better alternative in the same class from another rental place. In truth, it was only about 20 minutes, I was 1.5 hours early for pickup, and I ended up with a Mercedes C-Class Wagon, which worked out perfectly. I actually loved driving that car and glad I ended up with it. Staff was very friendly and helpful. Only minor issue is that gas was not fully topped off and car was not cleaned (not dirty really, but just not freshly cleaned like others). Not knocking off any stars for that as I was earlier and again was very happy with the experience in the end.
Tip: If you have the option to get a car with a speed limiter, get it. It's an alternate mode for the cruise control and very helpful in reducing the likelihood of getting speeding tickets from the speed cameras that are all over Germany. SIXT does charge a 25 euro service fee if you get one, but that is standard for all the rental companies from the sounds of it. I got tagged once before I found that feature and figured out how to use it.