We had a great stay in Robertos apartment in January. As a group of four people two of us had to sleep on the sofa bed in the living room. It seems uncomfortable at first, but the trick is to lie down in the right orientation. The boiler in the apartment had some issues for a time, which where fixed quickly after short correspondence with Roberto. Roberto was generally very forthcoming and provided much information regarding the arrival and the surroundings. From the apartment you can reach St. Marco's Square quickly by foot, if you want to visit other museums though you have to walk for quite a while. Apart from the problem with the boiler the apartement was well heated (the air conditioner could be used as additional heat source). If you come in winter you should bring extra warm socks or house shoes though, as the floor can get very cold!
All in all we really enjoyed our stay and the quality-performance ratio of the apartment was very good.