I recently rented from SIXT. The initial duration was for 19 days.
My plans changed and I returned the car after only 4 days.
The agent told me that I would receive a refund (about $650) for the unused portion of the rental.
SIXT subsequently refused to refund ANYTHING. No explanation. Just a refusal. I tried to call their customer support line which was busy for days.
I WILL NEVER rent from them again and am spreading the word to ALL of my friends and acquaintances.
I view SIXT as thieves.