I did it from Florence. From Venice you should drive along the Alps, but not sure if you see them.
For the record I drove in about 15 different countries and found Italy to be the hardest. Could just be my experiences...
-1, didn't find it bad at all (could be because it was August and there was no one around).
Bad idea to have a car in Rome. We rented one on our way out of Rome. We stopped at
http://en.wikipedia.org/wiki/Villa_d'Este on our way out of rome. It was magnificent.
In Florence and Venice (obviously) there is no use for a car. We parked our car and just walked around. We found it useful because you could plan your own trip and not be meshubad to train times. Also allows you to go to other places which would normally be a real pain.
We visited Lake Como, going to the city called Bellagio which may have been the most beautiful place I've been.
We then went to Aosta/Mont Blanc. Really beautiful. Cool to have a snowball fight in the summer. Did the best white water rafting I've ever done.