Remember to refill the tank!
I reserved a standard mid-size from Avis for a week's stay in Orlando, but when I arrived they offered to upgrade me to a brand new Mustang for an extra $80. I thought it was a great deal, and it did not disappoint! We both loved driving that car, and there was plenty of time and space to enjoy it on Florida's highways.
Avis is not unlike any other car rental company, in that they require you to return the car with a full tank of gas. In our haste to make it to the airport on the last day, I forgot to do that. They did warn me on renting it that they would charge me to refill it if I forgot, but I didn't realize they would charge $9/gallon when the prevailing rate was $3.50! I've since checked around, and it seems all car rental companies gouge you for the price of gas if you forget to refill it - and I've also learned there are 2 ways to protect yourself:
1) Just remember to fill the tank yourself
2) They offer a "gas insurance" pre-pay to make it cheaper to refill it if you forgot.
I didn't know any of this at the time, because I never forgot to refill the tank before. But I tweeted my dissatisfaction, and Avis got back to me within a day and reduced the price of that gas charge by 50%, taking about $45 off the bill. They didn't have to, but I was obviously unsatisfied so they went the extra mile to make me happy. It worked!
So I'd have to say I was extremely satisfied with everything about their service, and would feel comfortable both using them again and recommending them to someone else.