I stayed around one month at Ulli's appartment. It is located in a fantastic area, everything can be reached by footwalk, many shops, bars, restaurants, close to Bosporus and cetral bus or ferry station - very central place and also safe from my perspective, especially these days.
Ulli was out for this one month so I couldnt spend time with her, but she was always available by phone and ready to answer any questions I had. All in all a great experience and fantastic place to stay.