There's not a lot of roads into the city, so there's tons of traffic and pedestrians - all tourists with the same idea as you. Once we entered the outskirts and were trying to park, we discovered that most of the garages pack the cars in like sardines and want you to leave your keys with them - no way! So it took some extra time and frustration to find a garage that we could park ourselves and keep the keys.
We then basically got on a city bus (which in Venice is a boat) and did a lap through the city. It was cool to see the city, but (1) it was extremely hot, (2) the city bus was dirty (go figure), and since we only got off the boat briefly, we only saw the area right around a major bus stop (not pretty).
I think taking a train in or somehow getting dropped off would have been better and then spending a little extra money on a private boat tour would have done the city a little more justice. I definitely would go back.