The apartment was simply great! It was spacious, comfortable, clean, quiet, had everything we needed, and in a splendid location.
Space: there were two of us in the apartment, but even for four people there is ample space, including two full bathrooms with showers. Everything was clean, there were zero problems with check-in, etc. There were nice touches all over the place. Very cozy. We even stayed in one of the days when the weather was bad. Has some perfect reading spots. All other amenities are there, even more than I usually expect.
Location: it has at the same time great location (5 mins from Istiklal and Galata tower, 5 mins from the Sishane metro station, 10–15 mins from Taksim and the ferry station), and it is completely quiet during the night. The bedrooms are on the "inside" of the block, so you can get a good night's sleep.
Host: all communication was prompt, and Meltem was amiable. She didn't have a new check-in the day we left and made no problems for us to stay two hours longer.
Overall: 5/5, will definitely stay there again when in Istanbul.