SFO
Booked: X3
Received: Choice (in App) between a 530i and an E350. I took the BMW. Nice car, hate the new touchscreen controls though.

There was a major snafu on return. The greeter called me by the wrong name, then said "I see you're returning 6 days early." Turns out Sixt had my vehicle checked out to someone else. I immediately started imagining all the different ways I could get screwed by Sixt here, from damage caused by someone else, to extra day charges, to who knows what else. Sixt on-site staff was not particularly helpful or sympathetic, seemed to feel it was my problem not theirs. There were several other customers in the booth complaining about system issues at the same time as me - one had received a "you damaged our vehicle" email while they were still in the rental!

I gave a terrible review in my post-rental survey. Sixt actually followed up with an apology and refunded 75% of the rental charge.