We stayed for a week at Melisa's place, and had a great time! This flat is in the center of Beşiktaş, which has a busy nightlife, so the area was usually quite noisy until 3-4 am. It is very nice if you want to go out as you are in proximity of countless bars, restaurants, coffee shops, etc. The building door can be a bit tricky to find as it is between two restaurants.
Regarding the flat itself, the wifi is quite spotty outside of the living room and the air conditioner in the smaller room was not really working at times, but it is quite spacious, nicely decorated, and comfortable.
Overall, if you're looking into experiencing the Istanbul nightlife, this is a great flat to stay in!