I was a bit worried about using Sixt due to some the really brutal reviews here on yelp, but I have to say it was an excellent experience.  I came into Phoenix late on a Wednesday night, and saw that the line was pretty long at the three small rental car places that share a booth.  As I walked up though, the lines were for the other two places, not Sixt.  I walked up and was out the door in my car in about 10 minutes.  The price was fair, about the same as all the other rental agencies.  Where Sixt really wins is that they have some very fun cars for rent.  For the same price as a Chevy Malibu at one of the other rental places, I was able to have my choice of a Mercedes CLA, or an Audi A3.  I picked the CLA, and had a shiny black Mercedes with sunroof, for a long weekend.  The car had about 2,000 miles on it and still had that new car smell.  Dropping off was easy, the whole experience could not have gone better, I will use them again.