We stayed at Ibrahim place for about 1 month. It took time for us to choose on which side of the Bosporus we wanted to stay. Retrospectively I can say that Kadıköy was a great choice for us.
Ibrahim have been extremely flexible during check-in, allowing us to check-in earlier than anticipated. He gave us plenty of information about the area and even helped us with the Istanbul Kart (Istanbul Card). He has been super responsive to our requests during the time of the stay, usually responding in a few minutes, always with lots of detail.
The apartment is located in Kadıköy, on the Asian side. It's on the 5th or 6th floor of a building but it was never an issue for us. The area is vibrant and the main streets are very active with lots of people and lots of food options (as well as coffee shops and bars). This is a small and affordable grocery store literally just in front + a few more options less than 5 minutes walking. We ended up eating out most of the time because food is just everywhere, cheaper compared the European side, and various in terms of choices. There is a long walking area along the sea that you can also reach in less than 10 minutes (in either direction). This is very peaceful and nothing truly beats it on a sunny Sunday.
Going to the other side of the river consists on walking approximately 10 minutes to the ferry port and take a ferry. Usually a 20 minutes ride + usually around 5 minutes waiting time (there are a lots of ferry and we never waiting more than 10 minutes). This is super practical + also a very nice experience for us compared to a bus or a metro. We never got tired of ferry rides. At the port is also a ferry going to the Princes Island (takes 1 hour), which is a beautiful area too - you can do a day trip, watch the sunset from the top of the hill, go eat near the port and take the ferry back, watching the city lights.
The apartment in itself was quite big compared to what we were used to. There are two bedrooms (one with a large bed, one with a smaller bed + a desk), which was plenty for us as a couple (we used the second bedroom as office space for our remote working). The WiFi worked well most of the time thanks to a repeater that Ibrahim had set up, allowing to get WiFi throughout the apartment. The living room is large too with a very comfortable sofa and a massive TV with a physical Blu-ray (there are some cool movies in stock) + Chromecast (to play YouTube video directly for example). The kitchen is it's own room and has enough space too. There was everything we needed - and even more as we didn't use all the appliances.
The washer and the dryer were a big plus and worked very well, allowing us not to have to brave the cold to get some laundry done. Never had issue with the shower either, with enough pressure and nice and warm.
I can't think of anything wrong with the apartment nor with the location. It was absolutely perfect for us and I think whenever we come again we'll choose the same area and the same apartment if it is available.