I actually had a good rental experience with this company.
Of course I was very worried because of all the bad reviews on Google.
My main reason for booking with Sixt, despite the reviews, was because I needed to take the car to Ostrava, 25 km into neighboring Czech Republic. ALL other rental companies charge €25-60 for crossing borders, but it was included in Sixt.
I booked directly on the Sixt website as I got a better deal than going through any price comparison sites like RyanAir, Booking.com and Wizz.
Because of all the bad reviews and reports about made up damages, I bought their own CDW for an extra €16/day. I also bought an extra full coverage car rental insurance on the internet that covered everything and also the things not included by Sixt's CDW.
The car was nice and fresh with some 15 000 km on the meter, although not the Citroen C-4 as ordered but a Toyota Avensis Hybrid. It felt a bit weak to keep up with the 140 km/h speed limit on Polish high ways.
Before leaving and returning, I parked the car in a well-lighted area of the parking garage to take pictures/videos of any existing damages. Fortunately I never needed the photos/videos. Bring a clean micro fiber cloth to remove any dirt and a flashlight (use the torch on another phone) might come in handy.
DOCUMENTS AND PARKING TICKET
Don't forget to return the vehicle's registration documents and the NEW garage ticket you get when entering the garage upon returning the car.
There is an Orlen petrol station just 500 m from the parking house. it can be a bit confusing how enter the gas station as you have to make a full circle and go back the way you came from in the roundabout. I made sure to fill the tank completely by filling some extra after the gas pump auto stops. So for me a good experience.