We have rented from Sixt at DIA a couple of times. In fact we just got back from a three week trip. Haven’t had any notable issues. If you know the general rental agency tricks, you are good. We had rented a X5 online, and the agent at the desk was trying to upsell us at exorbitant prices to a X7M. Between that and overhearing another agent/customer conversation, I knew they were out of X5s. So I kindly declined and told them I’ll just take my X5. Then came the admission, followed by a free upgrade to a regular X7. The return was uneventful, and after a quick inspection and fuel check at drop off, we were cleared and sent an email stating all was good. The rates seem to very good for the cars on offer. I wasn’t going to drive a Chevy Malibu for 3,000 miles over.
No matter the agency, I would document the condition and mileage of the car with good photos at pick up and drop off.