Overall: A great, spacious apartment with a terrific location and professional, responsive management. My rating is based on a few issues that occurred during our stay (see below), but some of these were quickly addressed and hopefully others will be soon too.
Pros:
Location is great. The apartment is extremely close to the Kadikoy tram stop that goes north-south (can take you to Sultanahmet square or toward Dolmabahce Palace), the ferry to the Anatolian side of the city, and to the funicular up the hill toward Galata Tower. There are also useful bus lines nearby. The whole Galata neighborhood is walkable and lively, and it is even possible to walk up to Taksim square.
Note that taxis may be quite difficult to get in evenings from this location, as drivers (even on Uber) will decline to take you places due to traffic -- perhaps this is true everywhere in Istanbul, but something to watch out for.
Apartment is quite spacious with full kitchen, comfortable sofa, air conditioning, ironing board and iron, etc. It is a 5th-floor walk-up above a cafe (the entrance is actually at a stairway inside the cafe), but we knew that and it wasn't a big deal. It is possible to get into the apartment at night by alerting the security guard inside the cafe, which is closed at night.
Meso, the manager of the apartment, was very responsive by text messages to any of our questions and also was able to arrange a car to take us to the airport on departure. Car service to the apartment from the airport is also available.
Some issues we encountered:
When we arrived, the room was not ready (although our arrival flight time had been shared in advance) and so we had to wait for a while in the cafe below the apartment. This wasn't terrible but it would have been nice to get to the apartment because it was an extremely hot day (no air conditioning in the cafe) and we had just gotten off a long flight.
The first night, the bedroom was very hot due to one of the windows being broken / not possible to close. This made it hard to sleep. Management fixed that issue the next day and delivered a fan to help with any further heat issues. Later, I noticed there was another window in the living room that was similarly broken (looked like it had been taped shut, but the tape broke) and high winds also blew another window in the kitchen open that I thought was locked. I appreciate the fast remedying of our problem by the management, but just wanted to note what we observed.
The receptacle for holding the shower head in the bathroom was also broken when we arrived. Management fixed this immediately the next day which was great.
The wifi was also unfortunately fairly spotty during our stay. We used data plans on our phones to connect to the internet as needed during these outages, but this might be something to be aware of if you plan to work from the location and need 100% internet connectivity.