I stayed for two weeks in this apartment and would definitely stay here again.
Pros +
Incredibly spacious with three bedrooms and a lounge, bright and airy in a great location in Kadıköy, quiet, fully equipped, excellent shower with perfect hot water, extra linen in cupboards, incredibly fast and solid WiFi, plus aircon in one room and a massive portable fan to deal with the İstanbul summer heat!
Nazım and Serkan are very easy to communicate with and incredibly friendly and went out of their way to accommodate our needs, including our late arrival due to plane delay and a very chill policy around check out time.
Cons -
As others have said - the cleanliness really lets this place down. Overall, the place had clearly been cleaned because there was no dust and the bathroom was immaculate - but rugs weren't vaccumed, strong cigarette smell when we arrived, towels, pillows and sheets are definitely clean - but old, stained and torn and the toilet room smells very bad due to the sink lacking an S-bend.
With a little bit of investment in some new linens and a more thorough cleaning process, this apartment could be a top tier choice - but as it is if you're not picky (or bring your own linens), it's a great choice for the price and I'd recommend it.