Taxi is not a common solution in Italy, at least not common as in other european countries like Ireland, England, Germany and Spain (just to mention those I visited). 

It is mainly used by businessmen and tourists who didn't want to waste time studying the other public transport solutions.

It is still an highly regulated/restricted field and there has been several strikes (an example [here][1]) when the parlament tried to change the current laws.

You can have an example of how much you would pay having a look at the fares of the [taxi company of Pisa: COTAPI][2]

From my experience you could have to wait a lot to take a taxi during taxi's peek hour (that is when other public transport solutions are not possible like very early in the morning and late in the night) while during the day you shouldn't have to wait that much.

You could experience some problems in finding a taxi in little villages (even those crowded by tourists) while in the cities the solutions will be far more.

  [1]: http://www.euronews.com/2012/01/13/taxi-strike-in-italy-in-protest-at-liberalisation/
  [2]: http://www.cotapi.it/index.php?option=com_content&task=view&id=4&Itemid=34