I used to. I don't anymore, but I should resume.
The times I have tried to use a confirmation number as proof of something it failed I reserved a mini-van at Enterprise and they had none. Oh, 5 other people in line also had confirmed reservations for a mini-van.
The very nice Enterprise shuttle van driver who then took us to Hertz was honest with us during the ride. He said this location accepts all reservations without any system to ensure they will have the vehicle available at the time of your reservation, or that they even HAVE that type of vehicle.
The second time was also Enterprise, different city. Airport location in St. Louis. It is listed as a 24 hour location, and our flight came in at 830 pm. Get to the counter, it is closed, so I called the toll free number. The corporate reservation center confirmed that is a 24 hour location, and said that I may have to take the bus out to the remote location to get my car. They called their hotline and got no answer. While I was holding, the agents for the other rental car companies said Enterprise always closes at 7 pm and is not 24 hours. I got a car from another company, and the next day got a call from the manager of Enterprise at the airport wanting to know if I still wanted the car I reserved. I told her no, and asked her what happened. She said the reservation should never been accepted because they are not open that late.