Cem was an amazing, friendly host. He and I spoke a little each time we saw each other as he is a great conversationalist. You should like cats (you're in Istanbul after all) because there are at least two cats who roam about the flat. They will try their best to enter the room, which I usually didn't mind. They are easy to remove if not desired (Cem will help if necessary but I had no issue). The cats are shy at first, but very friendly after a little time.
The flat is in the middle of a moderately steep hill - of which there are many in Beşiktaş - so be prepared for that if walking. Otherwise, it is easily accessible by taxi. Beşiktaş square is a few minutes away walking and from there your transit options are limitless.
The flat itself largely clean, which was important as I stayed during the COVID pandemic. Cem was very adamant about handwashing as well. The litterbox can get kinda smelly, but it's nothing a little air freshener can't fix. On the whole, though, I had no cleanliness issues.
All in all, I totally recommend it.