The staff from the counter to the return was incredibly friendly. My problem was that I had a pre-paid rental car but then had to change my flights. I was unable to talk to a real person, only attempt to make changes via the web and that was unhelpful. The local phone number kept redirecting me back to the national line that does not allow a customer to talk to a live person.
So upon arrival I had no car (unless I wanted to pay over $100 more) and had to Uber back to pick it up.
But the car I was provided only had 500 miles and was very clean.
Also, while other car companies had 4-6 people working their desk Sixt only had 1-2.