The flat is in the very good location - 5-7 min on foot to Kadikoy pier,
and a big bus station (incl. Havabus stop, which goes to/from Sabiha Gökçen airport), 10 min to a big shopping mall. Pedestrian area with lots of cafes/shops nearby. From Kadikoy you can travel by a ferry for sightseeing (Blue Mosque, Galata etc). The view from a ferry is amazing, you never get tired of it.
Like most of the flats in the older part of Istanbul (close to all the sights) it is small and has thin walls, you will hear your neighbours and they will hear you. Although after midnight it is quiet and noone disturbed me at night.
Kaya is a very good host, he speaks English very well, quickly answering all my messages. The flat had some problems in the beginning (as it was newly renovated), but they were quickly solved by Kaya's handyman Ahmet, who lives in the flat below and speaks Russian. Also, when I told Kaya that the pillows are uncomfortable for sleeping, his sister Ipek brought us new ones the next day.
This is a good place for 1-2 persons. 3 maybe a little crowded, but if you're on a tight budget, it's fine because there is a separate small room with a sofa.