We loved the location, central to all the sights but on a quiet street with a lovely view from the terrace. The view is actually better than the photos show. Nice big breakfast buffet with lots of variety. Free coffee from the espresso machine all day.
The best thing about Marmara Guest house is the amazing and generous service. Nina made good restaurant recommendations, reasonable prices and great food. Yagmur did everything we asked and more, patiently explaining directions to the sights we wanted to see. He even generously allowed us to leave some of our luggage at the guest house while we took a four day trip to Cappadocia, even though we weren't staying at Marmara again on our return to Istanbul!
Overall, this is a very comfortable, clean guesthouse with exceptional service, and I would stay there again without hesitation.