I've used Enterprise a couple of times without any trouble. We have a local office close to home so I use them when the car is in the shop, etc. They don't have much of a selection but the rates have been reasonable. Basically I pick them out of convenience more than anything. I rented a specialty Jeep through them in Durango once without any issues.
I use Hertz through work. I don't have any complaints about their vehicles and they seem to have a good selection at the airport locations. However, the last few times I've rented, they have been very pushy about upgrades. They hooked my the first time, as I thought they were giving me a free upgrade. In the end they soaked me (my company, actually) for the upgrade to an SUV.
On the plus side, I missed a stop light once in Ohio and got T-boned by a dump truck in a Hertz rental. It was drivable so I took it back to the rental place and they swapped keys with me, no questions asked. I have a #1 Gold membership and they covered the damage 100%.