I rented cars for business from this Sixt branch for almost two years, the cars were always full with fuel, never noticed any scratches on the cars and returning the cars was always smooth and fast, the only thing is that sometimes I got cars that were a lot better than the others, for instance one time I got a hybrid Audi Q3 with automatic transmission, autopilot, Sport package, and cruise control with distance sensor... , and one time i got a hyundai i40 with almost no options in it, they even wanted to add a standalone GPS for me at that time (at the end i refused to get that car).
I always rented the same class CDMR/CDAR, and I know they don't give you the exact car you booked online, but at least it should be something simillar, so one thinks if they really respect and follow the car classifications and price catigories they have or they select a random car and give it to the customer, and one should think twice before renting cars from sixt for private, cuz. it might be too expensive for what you realy get here...