Me and my spouse stayed at Umut's place for two months.
The communication with him was clear and easy - he was responsive and helpful. When we got there the door to the shower was broken. He sent someone the next day to have it fixed. When we suggested that install a curtain between the bedroom and the living room as a space separator, he had someone come in the following week.
The small office space upstairs was very nice since my wife was working during our stay. The bed was comfortable and the location was great - well located in Kadikoy in a lively part of the neighbourhood. Nice bars and restaurants all around, as well as pharmacies and grocery stores nearby.
It is so close to the liveliness that on Thursday, Friday and Saturday, the party on the streets can be loud and until quite late. This is perhaps the only warning we could suggest future guests. But hey, if you love to party, you're right in the middle of it all.
We were very happy with our stay and would recommend this apartment to travellers that want to enjoy Kadikoy and Istanbul.