This is a wonderfully space. It has a beautiful ambience, with everything you need. We were here in December and I can vouch for the heating as well. Catch a sunset from the balcony if you can. It's spectacular.
Communication with the hosts was easy and quick. The flat's co-host Onur, was always available and especially helpful during our check out. We needed a late check out while the guests coming in needed an early check out. Onur managed to help both of us :)
In terms of location, this is in Balat. It's a colorful part of Istanbul... We loved the marketplace behind the apartment with vintage shops and traditional coffee houses. Tons of great eateries and even department stores are at hand. You are also minutes away from the Church Of Stephen and the Fener Greek Orthdox College. We stumbled on both.
To reach the city centre, we used an Istanbul Kart on a local bus, which has a stop at the bus stand below. This was easy and convenient. We got a yellow taxi to the airport.