Has anyone taken a taxi or a private transfer from the airport to where they are staying in Rome (like a prebooked shuttle) i can't seem to find one for Rome.

I've taken a taxi-before I discovered the train (which is so easy and only about 11 Euro). I am not aware of any shuttle buses to downtown Rome. There are shuttle buses for the airport area hotels (like the Holiday Inn) near Fiumincino.

I second taking the train. It's in the lower level at the airport and it takes you directly to the main train station with no stops. From there, hop in a cab. Easy!

There are several car services that Fodorites have used and recommended for years. They are priced competitively with taxis and are very good value for groups (about equal to three express train fares).

A search using "rome shuttle" yields lots of strings on this topic. Frequent favorable mentions for: romeshuttlelimousine.com
romecabs.com
limoservicerome.com

I've used limoservicerome.com (not to be confused with limoservice(s)rome which is a rip-off company). There isn't much benefit these days to booking limo services since taxis are were made a set fee (40€ for 4 people including luggage) a couple of years ago. If you use a taxi, make sure you use an official taxi registered to the city of Rome. Taxis registered to the city of Fuimicino can charge 60€ and unofficial taxis will charge whatever they think they can get away with. The airport website tells how to make sure you are taking an official taxi.
http://www.adr.it/portal/portal/adr/...vo_in_taxi_FCO