My personal experience was very good.
Delayed a day I experienced no problems adjusting my prepaid reservation. The counter person, Angnita was super helpful
Great car, and all staff were very personable.
Note that the location is off site, about 2.5 miles from the airport.
It's a little tricky to get the shuttle, I would recommend a taxi if you are in a hurry.
I have had very good experiences with Sixt and look forward to my next rental with them.