Uber only exists as Uber Black in Italy (to compare to the US products) Siince you are essentially reserving a car, not sure how immediate it might be, certainly not like regular Uber.
FreeNow is the most recommended app for Rome, but AppTaxi and ITTaxi are also available, and more recommended for Florence.
You are basically just using the app to call a regular taxi, with the added benefit that you will know what the ride will cost, and your location and destination are known in GPS.. They likely work fine when there is plenty of availability, but less so during off hours or busy times. Having someone call a taxi for you, or going to a taxi stand works as well.
I suspect with the apps, response may vary, since it makes the fare less "negotiable" for the driver. The fare though using an app will be a bit higher than going to a taxi stand since they add a service or call fee.