I have only rented from SIXT in Munich and each time (about 12 times a year) it goes very smoothly with no issues in the last 5 years and over 60 rentals. I book on their website and have a SIXT membership so all my info is in their system. Now for the US not so much as they are not up to speed yet. I can go through the SIXT counter at MUC in 2 to 3 minutes, in Atlanta 15 even with everything reserved on line. SIXT is my choice in Europe, not so much in the US.