Driving in Sicily

Sicily is covered by satellites for GPS navigation like any other place in Italy.
If you plan to use your phone as your GPS navigator, make sure you get an app that works when the cellular data is off. If you keep cellular data on and you are using a US based phone, your roaming data charges will be very high.

Have used my trusty Tom-Tom 3 times in Sicily (and will again in November )without a problem. Never have used my cell becuase I keep the data turned off because of charges from Verizon, however, if you have a T-Mobile phone, I don't think there are any additional charges.
My son swaps the Simm in his IPad mini and uses it for a GPS rather than his phone.

We were in Sicily for 4 weeks last year and used our Garmin GPS so if you have a GPS at home with European maps you can take it and use it in your rental car. Did not use cell phone so have no advice to offer. One caveat and I assume this applies to all GPS maps - when you get into the small towns the GPS is not accurate on those narrow old streets. We got lost once and had to ask for help, but aside from that instance we had no problem with our GPS.

When I drive in central Italy later this year, I plan to use paper maps, as I always have.

In pre-GPS days we drove all around Sicily using Michelin and Italian (Auto Club?) maps, with nary a problem.

The best option is for you to buy an offline app (TomTom, Garmin, NavTeq) that allows you to use maps without relying on cell phone data traffic. They are not free, but well worth the expense and much, much cheaper than renting a GPS-based unit from the rental company (which has the complications of possibly being not up-to-date).