I recently went to Italy with my wife and 3 other couples. We relied on Sorrento Silverstar car service to shuttle us between train stations, airports, Pompei, and Positano. I would be hard-pressed to find a single way in which this company could improve their services.
They are incredibly quick to respond to emails for reservations, schedule changes, or any other inquiries you might have. They consistently arrive early, so you never have to wait and wonder whether or not you have a ride. The drivers are careful, friendly, and extremely flexible, adapting to your changing schedule/agenda/whim/etc. This company is a big part of the reason our Italy vacation proceeded so smoothly.
And if you're traveling with a group, it's cheaper to book with Sorrento Silverstar than it is to buy train tickets or pay a taxi fare.