It works well in my experience. I've filled in it and sent it off or had it sent off 'telematically' for me year after year, and they've never made me pay the
canone.
Instructions here
Agenzia delle Entrate - Schede - Canone TV - Dichiarazione sostitutiva canone TV
which links to the form here
http://www.agenziaentrate.gov.it/wps..._sost+_mod.pdf
which you have to fill in and send off (just folded, with no envelope), or get someone like a
comercialista to do 'telematically' for you.
The section you're interested in if you haven't got a TV, like me, is Quadro A. The other squares on page 2 are for people whose TV licence is already being paid for by somebody else, or who are heirs of somebody who had a TV but has died.