I have used Airport Shuttle for 6 transfers over the past three years, in all cases for transfers between Venice aiport and Isola di Albarella. The pick up has always been timely, the chauffeurs very polite and the prices as agreed up front. They charge the fee for the return pick up upfront, which gave me some concerns, but although they already had my money, the level of service or professionalism didn't decline. Airport-Shuttle.com deserves this positive review. I just booked a transfer from Siena to Florence. I hope that will be equally smooth. If not, I will let you know.
... 2. November 2011 ... Airport Shuttle continues to deliver excellent service at the Siena-Florence transfer referred to above.