Reserved a car online. When I got to the desk they gave a clear explanation about the rental conditions. The parking lot was just ten metres from the desk, so no wandering through parking garages like you have in other airports.
A downside was the car. We booked a small SUV (Mercedes GLA/Volvo XC40 or simily) and received a BMW X1 with a lack of some basic features. We had a car that didn't have cruise control and the navigation system didn't recognize any of the streets I entered, so I had to use my phone for navigation. Which I couldn't charge while driving, because there was no USB-outlet in the car.
On return it is a bit of hassle to find your way on the parking lot. But you can't blame Sixt for that,the blame on this one is for Katowice airport.