I loved staying at Mert's place, Mert is a guy with heart for the good things of life and you can tell by his apartment. It's a friendly, zen place amongst the bustle of Kadıköy, my favorite part of Istanbul. The apartment building is an old one, it has a lovely vintage Istanbul feel to it. No elevator. Mert's place on the third floor is spaceous and clean and has all things you need for a perfect stay.
Location is excellent if you want to be part of Kadıköy life for a while. As mentioned it's a short and lovely walk to the boat station, or to the Kadıköy shopping, dining and going out streets. Tram stop (also to the ferries and shopping center) is a few steps away from the door, but you won't even need it, just use it for fun some time. Mert was home when I got there, and on leaving I dropped the key at Melek's market around the corner. Easy peasy. I'd stay here anytime again if I could.