To be fair, the description says you will be accompanied by an "escort", not a guide. However, our escort said as little as he could possibly get away with. We didn't know what town we were in unless we specifically asked him. He even seemed reluctant to guide us with hand gestures in and out of parking situations.
So you have a working a car, and some stops in some cool towns, but nothing like every other "tour" I've booked in various places around the world, during which you are made to feel really welcome and usually have some quality interactions with the guide. It is what it is, but I don't think it deserves a high rating, just on this basis. As long as you know what you are getting, fine.
If it were only that, I would give it another star, but we paid for the pickup/drop-off, over 100 Euros for two 20 minute rides, and they simply abandoned us at their warehouse (at least in our perception--I really can't say if anything was happening in the background). Things happen, I know, but when we got back to the warehouse with our cars and scooters, the leader/representative (or whatever he was) never said a word to us--turned his back and started toward his personal car. I chased him down and asked about my return ride and he said just wait here 5-10 minutes (in about 4 words). Then he left. It seemed kind of obvious to me that no one knew we were there waiting (all other guests seemed to have their own transportation and were gone) and after a half hour we were getting pretty anxious--we were alone at a warehouse in a little dingy office with really not much of an idea of where we were or how to find a taxi.
I had trouble getting the tour operator's phone number to work--I think there is a typo at one location in the operator info--but finally got in touch with them and the lady got someone there in about 10 minutes to take us back. So it worked out in the end after waiting 45 minutes or so. I disputed the transfer charge with Viator (at Viator's suggestion) and was denied, the operator saying that it was a traffic issue, out of their control.
Again, things happen, but the fact that we were just left there with really no idea what was going on, along with the nearly total lack of communication even during the tour, adds up to something I wouldn't do again.
Re the cars, they are what you would expect. A little oil smell, a grabby clutch, no turn signals, no seat belts, etc. All part of the fun and it always started. However, even if you have driven manual transmissions, this one takes a little time to get used to. I wouldn't want to learn on it, but I enjoyed it.
Also, realize that the pickup/drop-off is not to your hotel, but to a location in Siena that was about a 20 minute walk for us. The Fiat ride itself starts and ends in Badesse, a town near Siena.