The location of Mario's apartment is so convenient to seeing the sights in Venice. It's in a building just steps from San Marco, and is maybe a 5-8 minute walk across the square from the nearest vaporetto stop. The apartment itself is clean, comfortable, and well decorated. It was just two of us, but another couple or children could stay on the fold out couch, and there's a little living/dining room area and small kitchen. We really enjoyed our stay, and the check-in process with Mario's colleague Paolo was easy-- he was very responsive.
The apartment is above a busy street and restaurant, but we weren't bothered by noise. (Most of Venice seems to quiet down around 10pm anyway.) If you're a light sleeper or aren't used to city noise, maybe it's a concern, but we barely noticed. We were too busy enjoying Venice and the flat's convenient location!