We had a great experience with Sixt until the day of drop-off.
The good:
* The agent in the office (Princessa ?) was great to work with. She explained everything and even gave us a free upgrade.
* The car was in perfect condition and we didn't have any problems with it.
* I think the rental was reasonably priced when compared to other rental agencies.
The bad:
* We were doing an early morning drop-off and we were assured multiple times the shuttles start running at 4:30 AM. We arrived at the rental agency to drop off the car at 4:45 AM. The shuttles were parked in front of the building with no drivers. We waited until 5:30 AM and we were forced to get an Uber to get to the airport. Sadly, we were not the only people inconvenienced.