Originally Posted by
Philatravelgirl
I booked online with Italio and got great deals ahead of time and could pick my seat. Even a few days ahead was flexible enough. If you care about seat location on fast train I'd book ahead
Italo's a good option to look at. It's changed for the better, where before they didn't always serve the "main" station in each city. At least now, they do serve Roma Termini, Firenze Santa Maria Novella, Venezia Santa Lucia, etc.
If you're looking for a fast train directly from FCO, Trenitalia is still the only option.