Me and my girlfriend stayed for three nights at Jack's place and we had an awesome time.

Location: Jack's appartment is very central to the biggest highlights in Istanbul, you can walk to Taksim square and to the Galata bridge (to the Golden Horn) in just 5/10 min. Also, the view from his appartment is amazing, with the Bospuros and Hagia Sofia/Blue Mosk right in front of you. Eventough its very central his appartment lies in a quit neighbourhood, Cinghangir.
Appartment: Nice, clean rooms with antique furniture. The bathroom has a great shower, if it works, there was some trouble with the waterpressure. Hopefully he can fix that soon. Bedroom is basic, but the bed itself.... heaven!
Jack: Really a great guy. Made us feel very welcome. I could always get in touch if we had questions. He also offered us to join him with some friends to get some drinks, and he took us to an amzing little place with some of the best Turkish food in town. You won't find these things in your tourist guide!
So conclusion: Very positive. Jack and his place where a big part of a succesful trip!