Overall we had a great experience with Sixt at Denver airport.
Yes, their location is of airport and you need to take a shuttle, but the same is true for all other rental companies in Denver. Our shuttle got their a minute after we left the airport and when we returned the car their was a shuttle waiting. The driver even helped unload the car and stored away our luggage on the bus.
The car was in decent shape, it had about 27,000 miles and the typical rental car scratches, dings and curbed wheels.
The pickup process was very nice, the person at the desk was nice and offered us some water. They had the exact car that we booked and we got in the car within 10 minutes.
The return process was fast and easy, we just got asked if the e-mail, the receipt will be sent to, was correct.
The reason I didn't give 5 stars was that the car wasn't in the best shape and it had worn tires,and I expected more from a rental company that mostly rents out luxury cars.