This was our first Airbnb experience, so didn't know what to expect. Venice is a wonderful city, and we loved it. So the place you stay at doesn't matter all that much. Massimo was very helpful, and very organized, and very friendly. He even picked us up from the dock (Fondamente Nove). The apt is located within 7 min walk to Rialto and St Mark's. Also it's quiet here unlike living right next to Rialto. The apt was on 2nd floor, does not have an elevator, but then in Venice, you should be ready to walk a lot anyways. The apt was spacious, all utilities were working fine. It's a very basic place, and well priced, but nothing fancy or artsy about it.
However, my overall recommendation to travelers from developed countries like USA is that you are better off staying in hotels - far better customer service, fewer hassles, infrastructure like water, electricity, A/C etc always works, it is safer, usually u get a buffet breakfast, bed and bath is nicer, and you get daily housekeeping. Our experience with Airbnb in cities in Italy, Spain and France has been poorer than hotels in these same cities on earlier trips for the same price. Most airbnb listings are in old properties with crumbling infrastructure, and run by amateurs relative to large hotel chains.