This apartment has just been renovated: it is brand new, spacious, super clean and fresh. It was a pleasure to stay in Mehmet's property. The location is superb as well: you are basically in a couple of minutes from the metro, tram stations, Istiklal.
I stayed in Mehmet's apartment during the weekend which may have biased my view. As there are bars just below the window, it definitely gets loud inside in the evening and at night. One of two bedrooms is kind of soundproof as it has no windows — it is pretty quiet there regardless of the noise outside. The other one is not — if you plan to have a good night's sleep and you are a light sleeper, chances are that you find it hard to fall asleep till 2-3 am.
Other from that, there is virtually nothing to complain about. Mehmet and his friends are super responsive and helpful. Apartment, furniture, location, infrastructure — everything is very comfortable. Highly recommended, I'd definitely consider this property again during my next Istanbul trip.