Efficient service: The guy who handled the reservation was friendly and enthusiastic. He offered the excess insurance upgrade as you would expect, but once I confirmed that I already had cover, he accepted that.
The car was an Audi A5 convertible and was absolutely perfect for an excursion into the hills with the top down. The Sixt app had a detailed record of all the pre-existing minor scratches on the car, which was useful when checking the car over at the start and end of the rental.
Car return was fine, following the signs to find the Sixt section of the underground carpark. The man their was helpful and quick to process the return. Within twenty minutes of returning the car, I received an email confirming in writing that everything was in good order and no new damage. The rental wasn't the cheapest, but booking well in advance and going multi-day brought the cost down. It's a premium product, so I'm satisfied with the value for money.