Am renting a car in Florence with GPS unit to tour Tuscany for a week, has anyone encountered any problems entering in the streets or cities there?
Avoid the city center in Florence. It's not marked any better than the rest of the roads but they have cameras to capture your license plate and charge big fines if your car isn't locally registered.
The nice thing with the streets in the hill towns is they don't change much, which means your GPS should get you around nearly flawlessly. The GPS can be programmed to take you directly to public parking. The exceptions are construction or other obstructions that wouldn't show on any map.
Florence itself was the only place we didn't take our rental car during a trip in June. In the other places our GPS worked well and found us the parking lots mentioned in RS book easy enough.
Though the GPS was not without fault. It might be because our model is already 2 years old and misses are newer roads out there. Especially in mid-sized towns that built new ring roads with round-abouts to direct traffic away from the historical centres. But in combination with reading the street signs we had no major issues. And despite what others might have written here we did find the street signs quite helpful.