If you are considering renting a car to travel between the main cities (Rome, Florence, Naples, Venice, Milan, etc.) don't do it - take the train!
It is way more relaxing, faster and likely less expensive - gas, toll roads and parking are all very expensive in Italy.
Renting a car is great for exploring out of the way places but in doing so it is recommended to rent for a day or two from a main city; for example, Florence or Siena in order to visit the amazing towns of Tuscany.
Full disclosure - I am the CEO of ItaliaRail but even if I wasn't this would be my advice!!