I prepaid for the Mercedes GLC or equivalent through Priceline before really looking around (350$ for 6 days of a premium SUV? Sign me up) and a couple of weeks out I looked up reviews and saw that they were mostly negative so I was extremely anxious upon landing.
The whole process was quick, from arriving at the rental agency to driving off took maybe 10 minutes. I received a 2017 Cadillac XT5 (which I prefer over a Mercedes anyway) and it was clean and had a full tank. Dropping off the car was a breeze and I got my receipt via email a couple of hours later. The whole process was smooth, quick, and easy.
I would definitely recommend Sixt San Diego to my friends and will absolutely use them again next time I come back and need to rent a car.