Unfortunately, due to the governments plan to reinforce the building against earthquakes we could not stay in this apartment. Serkan offered us to either cancel or reservation or we could stay in his other apartment: https://www.airbnb.com.tr/rooms/1252352?preview.
We accepted to stay in the other apartment so this review is more about the apartment in the link instead of this one, but they should be in approximately the same neighbourhood. Serkan also accepted to offer free transportation from and to the airport in order to reduce the price differences between the apartments In short: Would I want to stay here again? Yes! Long answer: I don't have a lot to add to what everyone else has already said. The communication with Serkan went very well. His colleague escorted us from the airport to our driver that would bring us to our apartment. The driver he had arranged for us, Ferit , was a very nice guy as well. He gave us a lot of tips and the ride was comfortable even though this is Istanbul we are talking about!
Everything went smoothly and when we came to the neighbourhood another worker got into our car to bring us exactly to our apartment and to give us our keys. (We got 2 sets of keys!! Which was very nice since we would be staying with the 4 of us). The apartment is exactly on the first floor which allows for easy entrance (this is especially nice if you have been climbing the hills in Istanbul the whole day). The apartment itself is not very big but the 4 of us had a comfortable stay. I had to sleep on the sofa and I must stay that it was more comfortable than I had initially expected. The apartment was clean, well equiped with all the necessities that you might need. The only downside was that the stick of the toilet brush kept coming off so it would be really great if Serkan could replace that and that the TV did not have TRT1 (Turkish channel), because I would have loved to watch some football games :). Also my parents had some difficulties with the shower but for me it was all right.
Even though it is very central near Taksim and Istiklal caddesi which is of course very touristic and crowded, the neighboorhood (Cihangir) is very quite, and there are a lot of cats :)).
There are only a few hills that you may need to climb but the apartments location is excellent with easy acces to public transport and in case you need to take a taxi those also pass by frequently in a street nearby. In the street you can also find a convenience store, a grocery store (only vegetables and fruits) and a nice small cosy coffee place and a restaurant (Ulger sofrasi) that make homemade style turkish food and offer breakfast. If I need to stay in Istanbul again Serkan will be definitely the first person I will contact.