I spent one month there, and I'll start with the negative aspect of my stay: the studio is NOISY. Located above a nightclub (open every single night until 3 or 4am) and the walls are paper-thin (you will hear every doorslam in the building) Usually the street is very quiet in the morning, so if you can sleep late it's nice, but on my last week, there were construction works in the building, and with the bassdrum every night, I couldn't get proper rest, at any time.
So if you're not easily bothered by that or you're in Istanbul for a couple of nights out, no problem :)
Also the place seems a bit humid. It wasn't much of a problem in October but I don't know when the weather gets colder.
That said, the positive: the studio is big enough, nice bed, wifi and all. Two people can share it if you don't mind a little promiscuity.
And it's VERY well located, close to everything: metro, boat, restaurants, bars, supermarkets…
Roberto is a very nice host, easy to contact and he will do anything to solve any of your problems as fast as he can :)