I will write my review in two parts. The first part concerns the apartment, and the second one the host, Kemal.
The flat: This is a nice, renovated, and fully equipped flat in a working-class part of Istanbul. The neighborhood is safe and full of utility shops. You can get your fresh bread in 100 meters, get your grocery from a variety of local shops and chain supermarkets, and find ATMs nearby. You have many options for public transport: the bus stop is just 150 m away where it can take you directly to Istiklal street and Taksim square, the tramway T1 is ~900 m away and the Marmaray subway is ~1.5 km away. We just used taxi when we had senior people staying with us and could manage all traveling with the public transport ourselves. Importantly, you are very close to the Olivium shopping center which has great options and all famous brands. You can also walk to the Zeytinburnu Sahil Park and see the Marmaray sea (~2 km) and if you have kids, you can take them to many parks around, including the Zeytinburnu Millet Bahçesi. We stayed in the apartment for two months for our long summer vacation and I will definately come back here.
The host: Kemal is a wonderful person and we were lucky to find him as our host. Kemal is truly helpful, attentive, kind, and relibale. He kindly picked us up from the airport free of charge, and was 24/7 available for all of our concerns. For example, our parents were staying with us for a brief period and once they were lost in a nearby street. They could only speak english and called Kemal. He helped me to find them. In another occassion, we were locked out the apartment. He drove across town and helped us open the door. Importantly, he answered all of our questions about Istanbul and where to get the best things. In sum, you will not only rent a flat in Istanbul but will probably find the best host you have ever had, and will make a great friend.