We recently had a car reserved with RomeShuttleLimousine (who we had used successfully in the past), but our flight got all screwed up and we got in 6AM the day after we were supposed to get in. I had emailed the car service, but when we got to FCO no one was waiting for us, so we took an official taxi. Actually cost less and was just as quick. Luckily we knew where we were going as we had stayed at the same apartment in Trastevere in the past.