Deniz Apartments (Diamond) was overall phenomenal. To begin with, the host (Hatice), went out of her way to accommodate our early arrival/late departure. Not only was she there to welcome us in our "home in Istanbul", but she also remained in communication with me prior to our arrival at the airport to make sure all was going as planned and, once we landed, she coordinated our drive from the airport with our taxi driver. For this alone, the rating is worth the 5 stars above.
When it comes to the apartments, they accommodated all of us (four adults) very comfortably. All of the amenities listed online were as described, and there were even some nice extras (Laundry detergent, basic food items like tea and coffee, drying racks, books, board games, movies, etc).
Another plus was the fact that the place is located in a residential area of Beyoglu, which makes it very quiet at night in comparison with many other areas - we enjoyed full nights of sleep virtually undisturbed. (The comfort of the beds might have played a role in it as well...)
Although it may seem far from the old quarters (and most of hte toruists venues) on the map, we were surprised to realize how easy it was to go from there to the old city. With close access to the funicular (known as Tunel in Turkish as we found out the second day looking for it) and the tram makes for a very easy and scenic transfer to and from at virtually any time of the day/evening. We even skipped all this the first and simply walked to the he old quarters, which turned out to be, roughly, a 20 mins walk. The advantage of staying in Beyoglu however, was that it allowed us to enjoyed the old town by day and the quite vibrant night life (restaurants, bars, and stores) that the famous Independence Avenue has to offer by night while being close to "home" when we finally wanted our rest. In the end, we were actually glad we had chosen to stay in Beyoglu instead of the old town. After a hectic day in the over crowded tourist central of the old town, it was a nice change to wander the streets of the new, which offers a more local tone. Deniz apartments were a mere 5 mins wander down a street near the start of Independence Avenue. The other advantage of being off the tourist strip was that you will enjoy "local" prices for food and other items as opposed to the ridiculously inflated "tourist" prices of other location. On this, there are 2 local stores where you can find food within a block from Deniz Apartments, another bonus.
To sum it up, this apartment was extremely clean, location (for us) was ideal, it was in a quiet area, and the host was phenomenal. I highly recommend this place as it is well worth the price.