In reply to Frank the Husky:
We were there last September and used Avis. It was fine, with a three 'excepts'.
The first was that it took an age - over an hour - queuing up before anyone would deal with us. After a long flight, this really wasn't good. The desk was woefully understaffed for the amount of people waiting.
The second thing was toll roads. There are a number round Denver. We were offered, and accepted, a toll road pass. I forget what this cost, but it was worth it.
The third was at Denver airport on the return. Dropped the car off without issue, the bus dropped us off at departures no problem, but the signage at Denver airport is dreadful. It turned out that we had to go to floor 6 for departures but no-one in the bus drop-off area or anywhere close to it knew about this. We found it eventually by ourselves.
Nice part of the world, Colorado. Hope you have a good trip.
T.