I was originally very afraid of this outfit, given the negative reviews.
I am happy to report that most of these complaints are - seemingly - unfounded. Truth be told, all my interactions (from emails with the reservations group, to actually pickup/returning the car) where at par - if not superior - with Sixt's competition.
In fact, on the morning of 03/14 I walked into the downtown SF office (NB not the airport where I reserved) in an effort to pickup a day early. The young gentleman (I forgot his name) really went the extra mile. Unfortunately - and given the last-minute nature of my request - the price was prohibitive. Nonetheless, I left with diminished anxiety, and want to publicly laud this young man.
If you provide Sixt with enough notice, honor the conditions of their contract (times, documentation, etc), the price-to-quality ratio of their cars simply cannot be beat. It would seem that most of negative reviews came from those that didn't follow the above advice, can never be pleased, or fell out of favor with their CSR.
I am withholding a star. I was urged to return to Canada at the behest of my Government (there are/were talks about border shutdowns), after a single days use of a 9 day prepaid contract. I am very confident that Sixt will make an equitable adjustment, given the extraordinary circumstance of the COVID-19 outbreak. I will report back once I have contacted Customer Service; they are understandably swamped at the moment.