Mersin has a nice place, we stayed for 5 weeks.
Pros:
+modern, clean flat
+the flat has everything needed for a longer stay
+nice view and close to Istiklal-street
+Mersin and the assistant are very helpful
Cons:
-sometimes a bad smell comes out of the sinks
the buildings stairway is a bit dark and cats live in there (they are friendly though and cause no trouble)
-the next public bin is 100m down the street, so people leave their trash next to the front door (but it is being collected daily, that's ok)
Summed up: it's a nice flat in an authentic İstanbul-Beyoglu neighborhood witch has up and some downsides - pun intended - Istanbuls street are very steep:)