SIXT in my opinion are criminals.
I booked and paid for the car in full in advance - my plane was delayed so turned up two hours late - there was no car and the desk was shut and my keys were not left behind the service desk along with other customer keys. At the time I had two very young children and lots of luggage and it was midnight about 60km away from where we were booked to stay.
The next morning I called them and they basically told me it was my fault as I had not called and they would not leave the keys for someone who does not call - that was that...it was my fault, that I had paid for a car and was surprised when it was not there for me upon arrival. Two days later they gave me a (pathetic substandard car - I was expecting a VW Golf, I got a Opel Astra - they are not comparable).
I had paid for 7 days, so after 5 days they said my time was up (due t the two day delay in receiving a car) - and said the two days I did not have a car were my fault so I only get five as it was me who did not turn up. I kept it for the extra two days to cover what I had paid for then 400 euro was deducted from my card with no notice and to this day I have no idea what this is for and no one seems to want to tell me....
Finally, I had to extend my stay, for 6 days. I asked them to extend my stay as I could not be bothered to go through the pain of talking/meeting or seeing any from SIXT, I assumed this was the easiest approach. After 24 hours of making a request was told I cold have the car for 290 Euro...I mean come on! 290 Euro for an Astra for 6 days...That was nearly double what I was charged when I had originally booked.
This place is pathetic, the customer service is a sham, the supervisor was rude, and amazingly unhelpful.
Under no circumstances I would I use this horrid place every again... That is SIXT as a whole I would rather walk than go through the effort of dealing with substandard staff!
As someone who travels extensively and tends to get a car upon arrival - I am making it my mission to ensure that every one I know including people I don't steer clear of this underhand organisation