We rented from Auto Euripe and also used their web site to find our car and location in Rome, and then an easy way out of town.
We got the best rate by not renting from the airport, but by renting "in town".
We picked up our car on a Sunday, when not many rental lots are open. We got it at the huge underground multi agency lot under the Bourgesee park, adjacent to the Spanish Steps. They gave us a little cheat sheet that provided us with a quick and easy route to the freeways for our exit from town.
We bring our own GPS, which just makes things easy.
I have also heard that picking up a rental car at the Termini is easy. I don't exactly remember, but I think that we chose the Bourgesee lot becasue it was less money. If you use the AE web site you can play with the various alternatives to find the option that best suites your needs. I was amazed how much the prices changed with each option.