Just to provide a frame of reference on what Sixt's competitors offer in the customer service department:

Hertz FRA - Hertz gave me a S60 with Navi but no one knew how to switch navi to English. Agent didn't have another car avail but offered a discount for the rental on the spot. I drove the car from T2 to T1 where Hertz swapped with C class with Navi (which was what I had reserved) and let me keep the discount.

Enterprise MAN - Their website didn't ask for flight info. As a result, when I arrived, the car was in the other terminal (10 min walk). Agent called to have someone drive it over instead of having me walk. 35 minutes later the car still did not arrive. Manager immediately told me to return the vehicle without refueling it.

Hertz DUS Hbf - Reserved 5 series. No car avail. Lack of initiative in customer service department until I suggest they call the airport. Airport didn't have vehicle but Hilden did. Hertz paid for cab ride to Hilden location. After rental, I wrote the Hertz CS and they provided a $100 USD rental certificate.

This is the standard Sixt should be exceeding (especially because they charge more) if they are to be the best.