I spent a month in Omer Duplex with my little family (2 adults & 2 kids), so I think that I can give you a good description of the house.
The location of the house is strategic for sightseeing, If you have Google Maps, you will find it without problem. I took the bus from Sabiha Airport to Taksim and from there, you're at 15mn by foot, 10mn from Istiklal Street, there is a lot of shops around the duplex (bakeries, vegetables vendors, groceries, kebap restaurants, even places where to take your breakfast). There is a big park at 100m where kids can play.
The duplex is BIG in Instanbul standards.. it's literally 2 apartments in one. for 4 persones, it was more than enough (we didn't used the first stage), for 6 persons it's perfect I think !
The view is ASTONISHING, taking the breakfast with a view of Golden Horn and imagining the battle of Constantinople, all what happened between Byzantins & Ottomans, having a view on the Mehmet mosque and knowing that one of the greatest ottoman rulers is there is something which takes our imagination to another time in the middle age.. all the view is charged in history points and is really beautiful ! it's the PERFECT place to take the breakfast.
Internet Wifi is super fast, the kitchen have all the needed to cook, everything is new.
Omer is a very nice and helpful person! he greeted us despite the fact that we came at night, bought us water and was super responsive.
Each time, I tell myself that I would come back to such and such a place and I never come back .. but Omer's apartment is so great, that as soon as the checkout was done, immediately, I booked for another month next month.
If there is a strategic location for sightseeing in Istanbul and feeling in comfort, it is this duplex.