My partner and I spent over a month in Betal's flat and we truly felt like we were residents of Istanbul rather than visitors! The flat is in the centre of Cihangir and couldn't be better positioned with some of the best cafes, restaurants and shops nearby. The space is huge with a large kitchen and living area with an adequately stocked kitchen, 2 large sofas and not to mention a library full of books. There are also 2 bedrooms (one with a queen bed and one with a single bed) so there is plenty of space. Betal was super helpful and very responsive to any questions we had and he provided us a nice list of recommendations too.
We cannot recommend this place enough, especially for long stays and if we're ever in Istanbul again for an extended period of time, we'll be staying here again! Thanks Betal :D