We spent almost 4 weeks in Sicily in April/May. Rental car prices were extremely high. (For example, about 700 for the 5 days we rented in comparison to my upcoming rental for 3.5 weeks in France/Spain for about the same price, which is still high in comparison to past rentals) I love to be in the city center, and a car for the most part would be a nightmare in the cities of Palermo and Catania. We did note that having a car in Siracusa (Ortygia) would have been easy though it would have been parked the whole time, except maybe for a day trip to Noto. A car in Cefalu would have been doable but not necessary, and would have sat unused, unless we had a few more days and did some day trips. We definitely wanted a car in Ragusa and Agrigento areas and Randazzo, as well. We ended up just renting for 5 days in the southern part of Sicily and used a driver in Randazzo. This strategy worked well for us. The public transportation, in general, was good, and I'd say better than expected.
We have also been to Rome, Naples, Amalfi, Florence, CT, Siena, and Como and wouldn't have wanted a car for any of that. When we return to Italy for Tuscany, Dolomites, Umbria and Marche we will rent a car. No plan to have a car in Venice.