The concept and ambiance is great, and an overall good experience, the location is also great (but the door is a little bit not that visible, there is also no lobby, so ask the nearby restaurant waiters when you reach the location).
The good:
Clean, great location, in the heart of the Karakoy district, which is a very lively place, the maps are gorgeous (if you are into mapping or geography).
The not so good:
The standard room is very small (I wouldn't recommend it for anyone other than solo travelers, I believe deluxe rooms are larger), the entrance is not car accessible (not a major topic, the nearest point is around 50~100 meters away, but if you have large luggage, prepare yourself or call the hotel to arrange for someone to help you), there is no ground rooms or an elevator (if you are an elder or have special needs, be prepared to take the stairs often).