Mario was an excellent host. We did not have an international calling plan, and so we were not able to call and let him know when we arrived, but he was happy to establish a meeting time and place in advance to accommodate us. He is very friendly and an excellent communicator.
The apartment itself is exactly as described online. It is located directly on one of the smaller canals. The neighborhood was quiet but convenient to all of the major sites around Venice. It was a 5 minute walk to the closest vaporetto stop, which was a big help.
The only negative factor was the lack of air conditioning, but this was a minor inconvenience that was openly stated in the description online. I would not let it affect my decision to stay here even after having stayed in Venice on one of the hottest days of the year.
All in all this is a great place to stay, and Mario is a wonderful host. I would highly recommend it to anyone traveling to Venice.