EDIT: After contacting them on the provided e-mail (and insisting a few times), they made a refund in my credit card. So I'm changing the review to 4 stars.
They absurdly charged me extra for a "free upgrade".
I reserved (and prepaid) for a mini cabrio rental in Sixt Haidhausen. The cost of the reservation was 55,36 euros per day. When I arrived at the station they told me there was no mini cabrio available, but they would give a "FREE UPGRADE" to an audi TT, also cabrio.
Althought I wanted the mini, I accepted the upgrade, considering they made clear it was a free upgrade and the audi would also suit me. I also decided to purchase an entra protection to windshield and tires, wich costed an extra of 6,34 euros per day.
Few days after I returned the car, they suddenly charged extra 324 euros in my credit card, over what I had already prepaid. When I checked my account for the sumary of the reservation, I saw they were now considering a value of 109 EUROS PER DAY of rental of the audi TT (it was 55 when I reserved the mini).
So they charged me (a lot) extra for what they said was a "FREE "UPGRADE", and I simply can't do anything because they make you pre authorize 500 euros in your card when you collect the car.
I'm trying to contact Sixt support in a hope that it was a misunderstanding, but it's been 5 days and no response so far.
I'm deeply concerned and frustrated with the whole experience, and still waiting for an explanation (and my money back).