Here in the Veneto, the former Republic of Venice, the dialect is still very alive and well, and in many situations is more prevalent than Italian. And it really is quite different than Italian in many ways:
"She's a beautiful girl"
Italian: E` una bella ragazza.
Venetian Dialect (at least one version of it): Xe na bea tosa.
Lots of words change, and even the grammar a bit. Indeed, with many italian "dialects" it's quite likely that they evolved from Latin on their own, rather than there being some standard "Italian" that then diverged.
It's really amazing, too, how much it changes over short distances - I can't hear it myself, I don't speak it that well, but you can ride an hour by bicycle from here and people recognize that you're from over there...
Edit: another good bit of dialect would be appropriate for the comment by Mark_Book below, "xe un mona".
Another good example is Bergamo, a city on the side of the Alps, where people from higher part of the town has a different dialect from the people on the lower one.
On top of developing independently from the Latin, the dialects where heavily influenced by the different dominations each part of Italy had. North-west dialects for example have a strong French influence while in south Italy the influence is more Spanish.
Probably much more than 50%. I've never heard my grandparents speaking any italian at all, but I guess they could understand it to some extents.
Italian as a language started to spread among middle/lower classes only in the 60's when most of the people bought TV sets for their living room.
I still use my regional language (a variation of venetian) everyday with family and friends.
The above comment of mine received a fair number of votes with the majority being negative. The comment was intended to be humorous and in my view if people downvoted because they were offended on behalf of Italians they need to lighten up.
OTOH if they downvoted becaust they thought it was not funny, fair enough. I don't mind because it's not my joke I heard it on The Simpsons
"She's a beautiful girl"
Italian: E` una bella ragazza.
Venetian Dialect (at least one version of it): Xe na bea tosa.
Lots of words change, and even the grammar a bit. Indeed, with many italian "dialects" it's quite likely that they evolved from Latin on their own, rather than there being some standard "Italian" that then diverged.
It's really amazing, too, how much it changes over short distances - I can't hear it myself, I don't speak it that well, but you can ride an hour by bicycle from here and people recognize that you're from over there...
Edit: another good bit of dialect would be appropriate for the comment by Mark_Book below, "xe un mona".