We've also used Sixt quite a bit - have a loyalty card with them. We've used them in Switzerland and the UK and found Switzerland to be better - better cars and free upgrades. But after a bad experience at Edinburgh Airport we will never use them again.
We hired a car that had quite a lot of minor scratches all along the passenger side which were noted on the original rental form. When we returned the car they claimed there was a new scratch on the passenger door handle. We never did anything that result in a scratch and triend to defend our position at the time but we had a flight to catch. Well we got the bill for £445!!!! for a minor (1 inch) scratch on a door handle that we all know they will never get fixed - especially since they never repaired all the other scratches all along that side of the car. I know it is our reponsibility to check the car before driving off but there were quite a few things noted along that side with just general locations circled so it was hard to identify each one individually. Because we had no proof and had no reply to our email - we did pay but they have lost a loyal customer (we hire cars at least one or 2 weekends a month)!!!!
I was happy with Sixt up to then but really feel that they took advantage of us with this car. When they did the inspection they went right to that scratch as if they already knew about it. Our lesson learned is that we have taken out a separate car hire insurance policy that covers the deductible.
your comment "Our lesson learned is that we have taken out a separate car hire insurance policy that covers the deductible." ... are you able to pm me this insurance company that does this. I've been looking for something like this in switzerland as I rent lots of cars.