Heads up: you will have a great time here. The apartment is super central: you can walk to Taksim (and the metro) in under 10 minutes, go to the big shopping street in 15 or just hang out at the many restaurants/bars/shops/supermarkets in Cihangir itself (1 minute walk). Don't know where to go? Beril will definitely have a great tip for you: that's how I discovered Susam, art galleries and the best place to eat kumpir.
Some practicalities: the appartment is small, especially as you share it with Beril. Beril lives here fulltime and also invites friends or familiy over sometimes. However, because there are multiple rooms (little dining room and two living rooms) you can always have a place to yourself if you don't want to be in your bedroom. So for me this ended up being no issue at all as it never felt crowded to me. Also, overall I found Turkish people to be very kind and welcoming, and I was often invited to join during theses times as well.
I stayed for a longer period of time, as part of work instead of vacation and even for this longer period the size never was an issue. In fact, I even thought of it as home.
All the essentials are there as well: from towels to toilet paper, but also water as the tap water is not drinkable. If you come in winter, Beril will have extra blankets if you are cold. And there is also heating available, so the Istanbul winter is nothing to be worried about.
Beril speaks incredibly well English, communicating with her is also very easy.
I guess the only tiny downside is that Cihangir is a pretty busy neighbourhood, so that means that there is a lot of noise (both traffic and people) during the evening into the night. My solution was to just shut the window and that would muffle the majority (apart from the occasional shout, but nothing that would wake me up). For me, this was part of the megacity experience.
All in all I just had a great experience and if I visit Istanbul in the future for a vacation I hope this apartment is available again.