We used Romecabs.com twice for transfers to and from FCO and would use them again. They're very highly rated and reliable. We had a very early flight out of FCO (to CDG/Paris) and used Romecabs this past September. They are always on time, the driver is always friendly and helpful and the cars are always comfortable, clean and air conditioned (I couldn't say the same for some of the taxi's we used while in Rome). The cost of a transfer using Romecabs is almost exactly the same as a taxi. We were told there would be a small surcharge for such an early morning transfer but in the end, the driver told me not to worry about the surcharge which was very nice. The Romecabs driver insisted on helping us with our bags at no additional cost whereas the taxi driver that drove us from the train station to our hotel added an extra €2 per bag. You can book your transfer ahead of time via their web-site and you don't have to pay until the day of service. Have a great trip.