I had a wonderful stay at Derya's place. The apartment was clean with lots of windows and plenty of space, and was conveniently about a 10 minute walk from a conference I was attending! Derya happened to be away during my stay, but she connected me with the building's janitor who was waiting with keys when I arrived, and was happy to help me with any issues that came up (assisted by Google translate).
Her apartment is a little far from the old city - it took me about an hour to get there with a shuttle from the conference - so that's something to keep in mind. However, if you don't mind a commute, or you happen to be visiting this part of Istanbul like I was, I highly recommend Derya's place.