I had a flat on a Avis rental a few years ago and was about 7-8 miles away from the nearest location. Not wanting to deal with changing the tire or wait for Avis to send someone, I drove the car to the lot.

No hassles, they gave me that day for free, upgraded me, and didn't worry about the fuel level either. Frankly I was a little surprised with their nice service.

Another rental, Hertz in the Bay Area. Picked up a Taurus at SFO and problems the whole day with the transmission slipping. Was near SJC that evening and dropped by the lot. They exchanged the car with no questions asked. The replacement had the same problem, though.