For shorter rentals we have used this link (pickup Rome FCO airport) 5 times now, no problemos so far:
EasyTerra Car Rental - Compare the prices of car rental worldwide
Its a portal that checks the best rate at that moment from all rental companies (e.g. Europcar, AVIS etc). Works well, can be recommended.
BTW: Just love the Italian driving style being very close to the car in front :-) I guess Italians are so affectuous that this also reflects to driving habits ;-)