score:4
You might want to check opensignal.com, they have a map showing coverage - from that it would seem coverage on Sicily is below average for Italy; and I can tell you Italy in general is below average compared to other european countries.
That said, it seem I TIM (Telecom Italia) is the only provider actually covering the whole of Sicily, with Vodafone and 3 on second and third place and WIND coming in last.
sensorly.com shows an overall better coverage in Sicily, but the trend between the providers seems the same.