Short version:
The apartment is located in a non-turistic area of Venice, while still being just a short 15 minute walk from St Marks Square.
In sum, our opinion of the place / advice, is:
. good communication from the host regarding check-in / check-out instructions, and directions to the place;
. check-in is done via a lock box outside the apartment, so you don't have to physically meet Mario to get the keys. This is very convenient if you are arriving at odd times;
. check-out must be at 10am sharp. We left at 10.06am and Mario was just arriving as we opened the door to leave;
. the location is brilliant, withing walking distance to the main turistic areas (if that's what you're going for), but much quieter. Felt much more like a truly venician place where 'real' people live;
. the place is perfect if what you need a convenient location and somewhere to rest / chill-out before heading off again. Also good for simple meals, if you want to save on that instead of eating out;
. if you're looking for a modern "instagram-mable" space, then probably not so good for that;
. we would definitely consider staying at Mario's Apartment again;
. bring stuff to deal with mosquitos, if you can!
The longer version:
The windows in the kitchen and living room areas are facing one of the canals, which is really nice and cosy, especially in the evenings. We loved dining with the windows open, listening to the water. However, it can get a bit noisy at times because of all the boats passing by, but that's mostly during the day.
The apartment itself is dated, lightning is poor, and the kitchen and living room surface areas were covered in dust, which I believe is not due to lack of cleaning, but due to the nature of the building itself (we have cleaned it ourselves, only to come back home to more layers of dust). This was not a major issue for us, though.
The kitchen has all the usual appliances and utensils, apart from an oven. The dining area comprises of tall benches which were a bit uncomfortable, but ok if you're just planning on having a quick meal and then leave.
The major thing we felt was lacking in the apartment were mosquito nets. Closing the windows at night was not an option as, with no aircon, it would be impossible to survive the heat otherwise, so we were eaten alive by mosquitos.
All in all, a great place to stay in Venice.