We're satisfied with Sixt's services. The process was smooth, communication exemplary, and the car new, clean, and trouble-free. Our only complaint was nearly receiving a van with an empty AdBlue tank, which could've caused inconvenience, especially since we're only staying one night in Copenhagen. They filled it up after we called their attention.
For your information, the return section is one block away, different from the building where we received the car.