Definitely 5 stars!
Spent here 3 weeks for a family reunion during the New Year holidays. Great place! Walking (5-10 min) distance to Taksim Square, Galataport, local groceries and crafts shops, restaurants and coffee shops, etc - basically, everything you need to stay multiple weeks in Beyoglu/Cihangir areas of Istanbul. Only thing to keep in mind is lack of elevator in the building, so be ready to climb to 4th floor.
The place is super clean and renovated, no issues discovered whatsoever.
Mami is excellent host who was super responsive and helpful throughout the entire process. He helped to organize the transfer from/to airport, met us at the property to ensure super smooth check-in process and helped with the luggage. He event went extra mile and purchased a microwave oven upon our request.
Overall, I'm super satisfied with the stay and ready to recommend Mami and his place to everyone looking to explore Istanbul.