Similar to the other reviews, I had a wonderful time.
The apartment is absolutely amazing- great view, warm (it was winter), cozy, great shower/bathroom, comfy bed, good kitchen, etc. Indeed it was up 5 flights of stairs which may be a challenge to some but I didn't really mind- I think as everyone else had mentioned, the area is quite hilly so you should be ready for a workout in any case!
Ozgur is absolutely amazing as a host and he really did help me out in a few points where the trip went awry. My flight arrived late at 2 am and he was up at 3 am to facilitate my check in; my bag didn't make it to Istanbul with me so I gave his number to lost luggage and he was able to give me a head's up when the luggage was being delivered; I had a disastrous uber experience and he was available to explain to the driver where we were heading. I really want to thank him for making this a wonderful trip despite the different challenges.
The apartment is absolutely lovely and so conveniently located near a number of great restaurants/shops, so I would recommend this in a heartbeat to the solo traveler (or for couples)!