I came to Istanbul with 5 friends for 4 nights over a weekend. The location of the flat is excellent, and in fact I loved this whole area. In the morning, the city starts slowly, and you can sit outside quietly at little pavement cafes and watch as the city wakes up (Yaşar himself runs a small cafe nearby). In the evening, if you want to go out, there are scores of restaurants, bars and clubs within a few minutes walk. You will not be bored.
The flat is down a side-street, which means that it manages to be relatively quiet, despite the liveliness of this part of Istanbul. However, I perhaps wouldn't recommend it if you wanted a really peaceful, intimate holiday. For that, you might be better in an area like Çukurcuma.
Inside, the flat looks pretty much the same as it does in the photos. The rooms are a decent size, and comfortable. The flat was clean. Again, if you are looking for a 'boutique' AirBnB experience, this is probably not the right flat, but it's very good value, and we were all happy with our stay.
Yaşar was very quick to respond to our messages, and we had no problems checking in or out. He made the whole thing quite easy.