We went through this recently in Denver. We had a reservation with Thrifty, but when we arrived there was a multi-hour line to get a car (we moved maybe 3 spots in a line of 40+ groups in about 30 min.) To make it even better, the wifi was down in their building, or maybe it was above capacity since they really aren't set up to have that many people there waiting for their cars. I've certainly never experienced anything like that before. During that time I made a Hertz reservation, and so we walked across the lot, hopped in a nice SUV, and drove away.
That motivated me to book with Hertz for my MT trip. Pick up for that is tomorrow morning, so I'll report back how it goes later.