The only problem I have ever had with Dollar is their policy regarding credit card holds.
When you rent from Dollar, they put a $250 hold on your credit card, no matter how long your rental is. Even if you are only renting for one day, for $30 or $40, they put a hold on $250 on your card. My guess is that this covers thier own insurance deductable in case you steal or wreck the car.
I have rented from Dollar several times and been pleased with the service and the quality of the cars.