The most positive aspect about the apartment is its owner, Mark. He was very helpful from the very moment of our arrival - we easily found the apartment given all the directions. When we found out that the hair dryer was broken, he replaced it with another hair dryer the next day. Also Mark gave us advice on what to do and where to go.
The apartment itself, being located on the ground floor, is a bit dark, but it didn't really matter for us as we spent the most of the time outside. It's also noisy at night with the open window, but again, it didn't bother us much. We really liked the coffee machine and coffee + coffee filters provided by the owners. The location of the apartment is really amazing! You can easily get to the Taksim square or to Istiklal street, there are a lot of means of transport available: subway, trams, buses, etc. And it's pretty easy to get to the apartment from the airport (take a HAVAIST shuttle IST19).
There was one thing which we didn't discuss with Mark for some reason. We had only one set of bedsheets, and also there was no cleaning, though we paid for it while booking the apartment.
All in all, we highly recommend this apartment if you're coming to Istanbul to explore the city.