SIXT has a great fleet of vehicles available, particularly more luxury oriented offerings such as BMW, Mercedes, etc. My rental experience at the Denver Airport location did not start well, but improved... only thanks to my stubborn refusal to be sold short.
I had reserved well in advance, over 2 months in fact, a BMW 5-Series 4-door sedan. I even logged-in online the same day to "preselect" my chosen car, a white 5-Series that showed available.
Upon arrival, the agent who assisted me seemed preoccupied at first. She maneuvered around their office and various drawers, picking up different sets of keys and putting some back. She returned to tell me they didn't have the car I reserved available (even though I could see it in the parking lot). She tried to get me to take a Mercedes C-Class, which I refused, as it is smaller than the car I reserved. Then she tried a BMW X1, which I also refused (too small, lacks features and space). In the end, I waited approximately 20 minutes and refused at least three alternatives until I was provided with a satisfactory BMW X5.
The provided vehicle was great, and served my needs for this trip perfectly. I returned it on time, free of any damages, and full of fuel. The receiving agent was very pleasant. Our shuttle both to and from the terminal was great.
So a word of caution for renters: Do NOT allow any rental agent to downgrade your vehicle reservation and thereby overcharge you. It may or may not be a common practice, but I get the feeling it's commonplace for customers who do not know car models. Thankfully, I do.