Residenza is well worth having, not least because when you have permanent residency you are eligible to apply for a Carta d’identità, which you can flash instead of your passport when you are out and about in Italy. In my experience, in any dealings that require you to establish your identity, people breathe a huge sigh of relief when you produce a Carta d’identità and codice fiscale and having both documents speeds things up tremendously.
The procedure for applying for Residenza is given here. This is the official advice from the Italian state police, so who am I to argue? For what it’s worth, my own experience was slightly easier: I found that once I had bought a property and established a permanent address in the Commune, all I had to do was fill in a form at the town hall Anagrafe (the nearest UK equivalent is the registry office), hand over a few photos and wait for someone to come round to the house to establish I was actually living there.
Once I had done that, I then applied for my Carta d’identità at the Anagrafe and this also proved to be a very straightforward procedure even if you are not an Italian citizen. Once you have your residenza, you simply produce your passport (UK in my case) and a set of passport-sized photos. A permesso di soggiorno could be required in certain cases, but I have never been required to produce one since (a) Italy and the UK both became EU members and (b) I became a property owner in the town where I applied for my Carta d’identità. A small fee is also payable and once all the documents are in order, the Carta d’identità can usually be issued on the spot and is valid. I was initially issued a document valid for five years but it was an easy matter to renew for another five years.