My family and I stayed at Levent's for a month. Overall the experience was really great. The place is accurately described in the listing, it's clean and well maintained, the view is absolutely magnificent, the metro station and ferry dock are both just a 7 minute walk from the place which is fantastic, and the neighborhood's got everything you need -- all kinds of shops, restaurants and cafes.
Dr. Levent was very responsive, pleasant to talk to, and we had no issues reaching him at all. Our check in was very smooth and so was our check out.
A couple things to be mindful of though:
1- the apartment is on the top floor (4th) and there is no elevator so you need to walk up the stairs.
2- The neighbors on the lower floor weren't particularly excited about us (tourists) being there, my recommendation is immediately contact Levent and ask him to talk to them if they bother you at all.
3- If you're visiting in the summer (which admittedly isn't long in Istanbul) the bedrooms can get quite warm, although the living room has plenty of cool air breeze coming through the windows so I opted to sleep there :)
All in all, we had a great time, and would definitely stay there again if back in Istanbul.