Just used Sixt for a 5-day rental at SFO, overall a good experience.

First time using - they were about $135 cheaper than others that I typically use, and better cars than the Sentra-type cars normally seen with rentals. Will probably splurge next time.

As others have stated, they operate out of the Westin by SFO - note that drop-off is on the right-hand side *before* the gate into the parking lot. I accidentally drove past it as I'm used to going past some kind of barrier to drop off rentals, lol. But the attendant was able to take care of the return where I left the car.

I got a return receipt via email immediately; I checked the final invoice, available 48 hours later, for any surprise charges, but didn't see any.

Return to the airport is via the Westin shuttle, which takes you right to the terminal, so you don't have to backtrack on the AirTrain. The only issue is that I had to wait about 25 min for it. Not sure if this is typical or an outlier waiting time. Also wondering if the Westin shuttle would pick you up from the airport if you are not staying there, but just picking up a Sixt rental.