We had no problems with renting from Sixt, everything went quickly and smoothly. We had read about problems online when paying with a debit card, however we paid in advance using our MasterCard Debit (Monzo) with no problem. We also managed to find a bit of a reduction by booking on the Sixt Italia website too.
When picking up, Luca was very helpful, and extended our rent for 2 extra days at a cheap rate after our return flight was cancelled. He also gave a free upgrade to an automatic BMW 1 series, which made driving in the hills much easier. Pick up took just 15 minutes, and drop off only 5!