Yes we have done Rome on our own from civitavecchia and it was really easy...I did some research before we went i.e. got train times from Trenitalia website...but they are very frequent so really not necessary. You get the shuttle from the ship to the port gate and then walk for about 10 mins or so along the sea front with the water on your right. the station is on your left with a small car park in front of it. The tickets were 9 euros return last summer (which includes the metro fares) and the man will know what you want if you just say Roma!!! You must then get your tickets validated in the yellow (I think) machine on the wall/pillar in the ticket hall...don't forget or you'll have to pay a fine if caught! We then had to cross the line to the platform and caught the train going in the same direction that you walked in i.e. to the right as you come out of the ticket hall. It only took about an hour or so with nice scenery on the way. If you want St Peters and Vatican get off at St Pietro or go to Terminii (which is end of the line) if you want the colisseum etc. It was a Sunday when we went so we went right into the City. We then got the Metro to the Colesseum which was literally across the road from the station ...very long queues there so we then walked past the forum and eventually (stopping for something to eat on the way) to the Trevi Fountain and Spanish Steps from where we got the Metro back to Terminii. there are plenty of signposts from the colosseum area directing you to other sites of interest. I got street maps off the internet before we went. the trains were on time and clean and tidy, We got one back to the ship at about 3.00 to make sure we were on time...neither of us speak Italian and we found it surprisingly easy to do and cheap!!!!
No problems with doing Rome on your own just catch the train from Civitavecchia station and if you want to do the Vatican get off at St Peter's or go to the last stop, Termini, to do Rome itself. Plan your tour so that you get to see all you would like to, Rome has many web sites to help you, but do watch out for bag-snatchers and pickpockets, Rome is renown for them. Also it will probably be stiflingly hot so be prepared for this as well.....Great place to go but do your homework ....Nei.