I'm not really sure why there are such bad reviews about this place.
I heard about Sixt because my AMEX has a promo with them of 20% if you pay with your card. Well, I think the guy on the phone gave me the best deal anyway...I got a nice Toyota Camry for $25/day.
The process was quick and efficient. I'll definitely consider using them again if I need a car especially because they have a shuttle that comes and goes to and from the airport. How convenient!