The apartment is in a brilliant location - it is walkable to most tourist areas but also very conveniently located near to the tram, funiculer, Kabatas (Ferries to asian side / Princess Islands . Bosphorus tours), and next to the brand new Galataport cruise terminal (lots of shops and restaurants). The view from the apartment / terrace is stunning.
To echo some of the other reviews, Istanbul is very hilly and the flat is no exception - you will be required to either go up or down many steps each time you leave the apartment building (roughly 150 down to the main street) so be prepared for this. We found it good exercise but some may find this strenuous - we were assisted at check in with our bags.
The apartment building isn't directly on a road but rather little side paths so you will need to bear this in mind when ordering an Uber (Cihangir Mosque is a good meeting point).
Communication with the host was great and very helpful - Had a really great stay and can't recommend enough.