Overall, Sixt's services are top in my rental experiences in the past few years. I've used the several times, far more than any other rental company. No matter whether in Glasgow City, Glasgow Airport, or other countries in Europe. The only minor problem I experienced is the fuel policy. A case is that I chose 'full tank' policy since the staff said the fuel rate is lower than any regular gas station. When I got the car, I found the tank was not full but I still was charged the full tank quantity. Further, when I refilled the tank during the trip, I figured the full tank fee was only about 75% of Sixt's full tank price.
Since I had a bad experience with 'full tank' option, I choose 'like to like' option on another rental.
I took pictures of the fuel gauge while picking up and returning the car. No surprisingly, I was billed with extra fuel after 3 days I returned car. The extra fuel fee is a lot, the amount is enough to refill 1/4 tank of fuel. The gauge was at the full level when I returned it and it was impossible to pump so much extra fuel in. I knew it because I checked the tank size for the car model. I reported to customer service and showed the picture.
The customer service returned the fuel surcharge after I filed my complaint.
Unlike a lot of rental companies, each branch's service quality varies. It looks like sixt's customer service is more willing to solve customer's problems from every branch. That is an important reason I keep using their services.